BW could easily buff underpowered PvP dps classes by adding a +10% resistance to damage when stunned or cc'ed to the top tier talent for these classes. This would have virtually no effect on endgame PvE.
And how much effect do you really think that would have on their effectiveness in PvP? Commandos are worthless in PvP because they are functionally a turret class without turret protections or burst mobility. 30% decreased damage taken while stunned or incapacitated isn't going to solve that problem. You're suggesting a miniscule situational buff that, yes, while it affects PvP *way* more than PvE, isn't really going to make an appreciable difference in performance. If it *did*, you'd see a *lot* more Tactics VGs (that actually get a 30% reduction in AoE damage *and* 30% reduction in damage taken while stunned) running around in PvP.
PvP is a holistic balancing effort, substantially more than PvE is. Largely unquantifiable values such as mobility (Force Speed v. Force Leap v. passive +spd; the passive +spd buffs amount to 60% the same mobility increase as Force Speed but, because Force Speed is a *burst* mobility effect, it's *way* more useful; Force Leap lets you gap close faster with more utility and damage than Force Speed on a lower CD, but Force Speed can be used to escape; are they balanced or is one of them better than the other because of those differences?) and utility (how do you numerically compare the virtues of stealth to slows/immobs to CCs to stuns to KBs) have to be factored in right alongside damage (which is further muddled by the need to weigh burst DPS heavier than sustained DPS for PvP but not for PvE), survivability (once again, muddled by the fact that CD centric burst survivability such as Sentinels get is more valuable in PvP thanks to the short time frames of combat, not to mention the vast gulf between the usefulness of mitigation stats in PvE, where they are *amazing*, and PvP, where they are questionable at best), and healing (hardcast healing is substantially worse than instant heals because of the presence of players actually interrupting; in PvE. it's relatively easy to quantify the differences because of the 1.5 second GCD but, in PvP, where interrupts are actually present, a 1.5 sec cast heal is *not* functionally balanced at the same ratio as an instant cast heal, not to mention widespread pre-healing effects, such as double stacking the Scoundrel HoT or Sage healing the entire group, are substantially more useful than they are in PvE because you can't expect just the tank to take all of the damage for the group).
So, no, it's not as simple as you think it is. Most of the knowledge that top end PvPers have acquired is heuristic: they feel, thanks to their experience, that Sentinels are better than Guardians as PvP DPS, because they're harder to kill and hit harder, but they can't adequately quantify those values. Because those differences cannot be quantified effectively and some of them can't be addressed without also affecting PvE (such as reducing the effectiveness of the Sentinel survivability CDs which are balanced in PvE but way more effective in PvP; balancing them for PvP would end up making Sentinels squishier in PvE, therefore affecting that balance consideration), the only way to effectively balance the classes is with repeated cycles of balancing coupled with monumental amounts of playtesting and, guess what, the number of hard core PvPers actually willing to spend a lot of time on the PTS just to mess around with changes to various class effectiveness for classes they may not be good at played or enjoy playing is remarkably small (sure, you'll get a lot of people saying they would be willing to, but a vast majority of playtesters just hop over to see what the changes are and then leave without actually giving them the testing that is needed). As such, to reach said balance, you get lots of changes that make it to the live game that aren't actually ready for it because they weren't adequately vetted by the hard core PvPers that were needed to make sure they were appropriate.
Blizzard has had a *way* bigger staff than Bioware and a large number of them are explicitly focused on PvP and yet, for some reason (/sarc), there are *always* classes and specs considered worthless for PvP explicitly because they were balanced elsewhere. When I stopped playing WoW during Cata, Shaman healers were the *only* healer any ranked PvP group even dreamed of bringing in; Priests, Druids, and Palis were all considering medice non grata for competitive PvP. Sure, you could field one of the other classes, but you would pretty much always prefer to take a Shammy instead (I remember because I played a Shammy healer and got constantly pestered to PvP even though I didn't enjoy it when none of the Palis, Priests, or Druids in my guild who *did* enjoy PvP could find a ranked group willing to take them along). And, of course, this was also during the time frame that, for *PvE*, Shammy healers were remarkably underpowered compared to the other healers for 99% of the situations in game. The other healers weren't going to get buffed to make them viable for PvP because they were already strong enough in PvE, and Shammies weren't going to get buffed for PvE because they were already the best PvP healers.
It's *nowhere near* as simple as you seem to think it is. Just because you have a heuristic notion that some classes are weaker than others at PvP based upon your experience doesn't mean you have the ability to actually appropriately gauge what changes would be required to *make* them balanced *without* impacting their performance in PvE *explicitly because* your notion is entirely heuristic. You don't know how 30% decreased damage taken while stunned is going to actually impact the ability of a Commando to function in PvP; personally, I think that it would just make them spend slightly longer locked down by melee classes that just leap to them and blow them up before they end up dying and respawning just to have it happen again because it's not addressing the core weakness of the class. To make them effective, they either need to become utility gods (which I really don't see happening) or they need to gain some specific capability that will address the exact reason why they suck at the moment (i.e. they have to stand and cast to be effective but they have neither leap protection, interrupt protection, nor burst mobility that is required to be an effective stand and cast in PvP).
The games that have managed to create well-balanced PvP environs have accomplished such explicitly by simplifying things so that there are fewer variables to account for: League of Legends removed most sources of randomness (attacks all deal a flat amount of damage and almost all "chance for" abilities were removed so that it's almost purely predictable; the only thing that remains random is Critical Strike chance, which only applies to some abilities) while both Guild Wars and League of Legends restricted the number of abilities accessible at any one time to any class/champion to an exceptionally small number of abilities. Both of those decisions were pretty much explicitly made to make the game easier to balance for the purposes of PvP: fewer abilities equates to fewer variables and static damage with most abilities incapable of causing critical hits means that even the randomness is controlled and restricted.
If you want a game where virtually all classes are well and truly balanced for PvP, go to a game that is explicitly about PvP *and was designed as such*. The sheer number of abilities combined with the stat based differences caused by gearing (not just between tiers but even in the same tier with different gearing mentalities), mechanical differences based upon different specs (even 2-3 points moved in the same spec can have major differences in PvP performance) and the need to preserve PvE performance all add up to make cost-and-time-effective PvP balance for all classes and specs functionally unattainable except through some magnificent accident. In short, you don't get a lot of options for a class *and* balanced PvP; it's one or the other *especially* if there is PvE thrown in the mix as well because that just serves to provide explicit caps for attributes like damage, healing, and survivability so as to preserve PvE balance. Learn to deal with it.