PvP specific stats have become fairly pervasive and even ubiquitous in modern MMORPG gaming. The most recognizable example of this trend is the resilience stat in World of Warcraft. Age of Conan also contains a number of PvP specific stats. On the other hand, games like Warhammer Online do not have specific PvP stats, opting instead for gear sets optimized for PvP or PvE. Judging by the PvP specific stats seen at PAX, we speculate that one or both of these examples could exist in The Old Republic.
The reason behind the use of PvP specific character modifications is typically game balance. At the highest level of content it becomes extremely difficult to balance a game both for PvE and PvP. The most difficult NPCs and Bosses in a PvE environment tend to be far more powerful than any one player character. Conversely, in a PvP environment it is essential to make each player character as similar in power as possible at the most basic level.
Herein lays the problem many developers choose to solve with PvP specific stats and gear. Such a system allows players interested in PvE to build their character using PvE specific stats and gear and vice versa for PvPers. Developers are then free to build content in both areas without the major headache of having to balance both together. This division also allows players to focus on their area of interest. If a player is only concerned with PvP then they can progress in, and receive rewards specific to, that activity.
However, this division of stats and gear does come with its own set of issues; the most obvious of which is fragmentation of the player base. With distinct stats, players will generally need to decide which type of gear or stats to pursue. That decision then determines the play style they will be beholden to. This tends to concretely split the players into either PvPers or PvErs.
While players can choose to seek out both PvE and PvP gear, it is no simple task. This is especially true at the highest levels where gear acquisition becomes extremely difficult and time consuming. Also, open world PvP and PvP servers will still cause issues in a situation where someone has taken the time and energy to maximize their effectiveness in both areas. If you are PvEing in an area allowing open world PvP, what help is your PvP gear if it's stowed in your bag or inventory?
There is another, more subtle, issue that comes into play with PvP specific stats and gear. Since the beginning of MMORPGs, the amount of time and effort put in generally equals the quality of rewards gained. This manifests itself in many different ways from game to game, but is basically the same general concept throughout. You put time and effort into questing/grinding and you are rewarded with a more powerful character. You put time and effort into end game raiding and you are rewarded with better gear and access to more content. Where this pertains to PvP stats and gear is when player proficiency is brought into the equation.
When stats are unified then time and effort put into PvE becomes just as valuable as time put into PvP even though a PvEr's personal skill may not be on the same level as someone who spends their time focusing on a PvP path. This particular issue could be looked at as a pro or con depending on your point of view. Many players feel that progression and proficiency in one area should grant rewards in all areas. On the other hand, some players who progress through PvP and maintain a certain level of proficiency in that area may want access to rewards tied to both their specific progression and proficiency in said area.
PvP specific stats and gear can solve many issues that MMORPG's deal with, yet they bring their own set of potential problems. The idea and implementation is incredibly complex, as you can see from the brief descriptions above. Unfortunately, at this time, it is next to impossible to speculate on how PvP specific stats and gear will solve game design problems in The Old Republic. It is also difficult to hypothesize what new issues they may bring. As we move closer to launch, and more information is given, a much clearer picture of what to expect should emerge.