I'm curious if either of the players who are talking about PTs/Mercs have tried healing on their Mercs? In terms of PvP the strongest spec by far for Mercs is currently Bodyguard, then Pyrotech, then Arsenal. For PT it's Pyrotech, then Shield Tech, then Advanced Prototype. I play a Pyro PT (Valor 95), and a good Merc healer can stalemate you for 30s with a well timed bubble and either some badly filled resolve by your teammates or a little help with peeling or cross-healing from theirs. This of course entails playing to the skill cap of that class, since I only can recollect three merc healers who got that much out of it.
In general, the current trend is that each class has one, maybe two, viable specs for ranked warzone groups. I'll go through each AC based on my own experiences and highlight what I think those specs are:
Marauder - Currently the two best, and somewhat interchangeable specs, are Annihilation and Carnage. An Annihilation Marauder has the highest survivability of the three specs and still does some of the best single-target damage with bleeds. Anni spec is ideal for holding or capturing a node solo due to the self-heals and cooldowns, making a Marauder who plays to the skill cap of their class nearly impossible to kill by any other AC with the exception of Darkness Assassins. Anni spec is also ideal for controlling middle in a Huttball. A Carnage Marauder gains more burst and a bit more control (ranged root, ravage root), but the huge reason why they're so important for ranked warzones is the 80% Predation(/Transcendence) speed increase. A team without that speed increase is unlikely to get the Huttball first, cap their side turret first, or quickly move from door to door in Voidstar, putting them at a distinct disadvantage. Rage spec, in my opinion, is not currently suited for ranked warzones. It's a perfectly balanced spec with the most recent changes, but it's focused on AOEs with Smash and padding damage numbers that way. With a Marauder you're not looking for stats, you're looking for pressure, especially on healers with the heal debuff, and the other two specs offer more of that with comparable net damage.
Juggernaut - Juggs are one of the classes where all three specs are pretty well designed. Vengeance is probably the strongest Jugg spec at the moment, since you can put out damage in the ballpark of Rage, but you can focus that damage a bit more and additionally get to use your spam slow for free, as well as get 4 seconds of CC immunity after every Force Charge, which can be a huge deal for Huttball or for pounding someone with Ravage. Immortal would be second, since of the three tank ACs an Immortal Jugg is currently the most ridiculous to try and kill. They make ideal point defenders or huttball carriers and have pretty good peeling capabilities with their leaps, slow, stuns, and push. Rage also has its purposes as the high net damage spec, and a Rage Jugg provides more utility than a Rage Marauder generally.
Sorcerer - Madness and Corruption are the specs of choice for a Sorcerer, with Lightning falling behind in high mobility PvP. A Madness Sorc can keep pressure on multiple enemies and do large amounts of periodic damage (mostly) on the move. With Force Slow, the slow on Force Lightning, and the root on Creeping Terror, they also have a lot of ways to reduce the other team's mobility. Pairing a Madness Sorc with a Pyro PT or Anni/Carnage Mara is a good way to insure those classes are always in range of the target to use those skills and the focus is shifted away from the Sorc who's doing comparable damage when left alone. Corruption Sorcs strike a balance between inductions and instant/channeled heals, enabling them to be effective while moving and standing still. They bring utility in the form of Extrication and Force Speed, and have considerable CC capabilities with knockback roots and bubble burst stuns if speced hybrid into the Lightning tree.
Assassin - The only real spec for an Assassin right now is Darkness, though Madness also has its uses, and most specs involve some hybrid of those two trees. The closest thing to an "overpowered" AC/spec right now is Darkness, since it enables an Assassin to benefit from all the sturdiness of their tank cylinder and cooldowns, and the ability to guard, while putting out in the realm of 85%+ the damage of pure damage classes due to the use of damage gear. A well-played Darkness Assassin is an ideal ball runner, point defender/attacker, and peeler for healers. The current dilemma of ranked warzones is why you would bring any other tank spec AC when you can have an Assassin do similar protection and considerably more damage/utility. Deception spec is currently not very well conceived in my opinion, and Madness is not bad but it suffers from the fact Darkness is so good.
Mercenary - As stated before, Bodyguard is the best spec for Mercenaries. Pyrotech is viable in the right hands, but those hands seem to be few and far between. Arsenal can do a lot of damage but is easy to counter and more suitable for PvE. A Merc healer is the healtank of the three ACs, with a focus on powering through damage with an array of heals rather than dancing around like an Operative or using CC to get off heals like a Sorcerer. With a bubble up and full resolve, a Merc is basically unkillable for those 12 seconds. Pyro is all about finesse for Mercs and being unseen. They can stay at 30m and use many fewer inductions than Arsenal, so the basic idea is to try to be as inconspicuous as possible at your max range and kite, kite, kite. If they do that well they can provide high damage with the ability to cross-heal.
Powertech - The strongest spec for PTs is Pyrotech, followed by Shield Tech. Pyro PTs provide arguably the highest burst damage in the game and are competitive with Madness Sorcs in net damage. A good Pyro PT can time their skills to provide that extra punch needed to kill a healer or other important target and provide some utility with pulls and taunts. The role of the Pyro PT is always offense, except in huttball where they make better middle controllers and defenders. Shield Tech can be useful in a support role, and a good Shield Tech can be consistently hard to kill and a nuisance to the enemy team. Juggs and Assassins still provide better overall utility in their tank roles, but a Shield PT can pull their weight in that capacity. Advanced Prototype is more or less a spec in need of refinement, though it has some gimmick uses like Hydraulic Overrides in huttball if the enemy team doesn't control resolve filling.
Operative - The strongest spec for Operatives is Medicine, followed by Concealment. Currently Medicine Ops are the strongest overall healers in the game, both in PvE and PvP. Their ability to use the majority of their heals on the move and free heals on targets under 30% health makes them very difficult to take down, especially for enemies who don't understand the class. They also have the benefit of being able to stealth, and stealth allies every so often, which lets them get out of tight spots and enables them to defend points by sapping enemies and living a long time. Concealment has been nerfed quite a few time since the days of yore where an Operative could 3-shot you from stealth, but it can still be made viable if played tactfully. Opening on someone is still the cornerstone of the class, which does make it less ideal for the set piece prolonged fights that tend to happen in competitive ranked matches.
Sniper - Snipers are currently (and finally) pretty well positioned in all three specs. Marksmanship provides the highest overall damage and the highest burst damage of the three specs. If a Marksman can set up shop in a good location where they always have priority target in line of sight they can cause considerable problems. Engineering is more about AOE damage and utility, with the ability to spawn Plasma Probe on a location indefinitely so long as they're alive, which can largely prevent point capture. An Engineer also is a bit more sturdy than a Marksman. Lethality is the more traditional PvP spec of the three, since it offers high mobility at the expense of less net damage and the ability of other classes to cleanse off the damage. All three specs are in a pretty good place right now if played correctly and in a suitable team composition.
So should the specs not listed be balanced more? Probably some, though not necessarily for PvP only usage. In most cases they have PvE applications, and it's not a crime to make certain specs ideal for PvP and others ideal for PvE. Currently Arsenal Mercs fit that role of being good in PvE after the fix for Unload. AP PTs, Lightning Sorcs, Lethality Ops, and Deception Assassins may warrant some changes to make them ideal in either one or the other, since currently they're all outperformed by another spec of that AC in both PvE and PvP.
For Operatives it's mostly a case of undoing some of the nerfs that were put in place back when there were more damage boosts available in PvP. For PTs it's about making AP a difficult rotation to sustain on the move but a higher net damage on boss-type mobs. For Lightning Sorcs they need to change the mechanics some, but as a spec it's within 5% of Madness so not terribly far behind. For Assassins it's really a matter of divorcing the damage from the Darkness tree as they did in 1.2 with Powertechs and the "carolina parakeet" builds, since if you don't make that spec do less damage it doesn't matter if the others do slightly more since they have considerably lower DPS uptime from survivability.