What you're basically saying is "don't play Gunnery/Arsenal in PvP". Which is stupid. Every spec should have something to offer in PvP as the tree's description says "Damage", not "PvE". Watchman is probably the best DPS spec in PvE - it's also very, very effective in PvP. Certainly a lot more so than Gunnery. Snipers/Gunslingers also have three perfectly viable trees. Rage/Focus got buffed to make it better in PvE so now it's equally good in PvE and PvP as well.
I agree, however, that Gunnery/Arsenal doesn't need extra burst or leap immunity. A brief immunity to interrupts, for example, wouldn't affect PvE at all, neither would more escape abilities (which fit perfectly with the lore, btw: a Mercenary fights for money but his own life is more important than that; a Commando is an elite soldier who must be able to evaluate the situtation and know when to retreat).
Rage is out of balance and hopefully not the barometer by which other classes' effectiveness should be judged, and engineering is terrible period.
Apart from marauders, just about every class has a spec that isn't effective in pvp. For sorc, it's lightning. For assassins, I'd argue madness is pretty weak. Tactics Van is immoble and would require a server client synch that's just not present to be effective in pvp, so everybody goes pyro. Vigilance wasn't great to begin with and just got nerfed. Lethality ops are far inferior to their sniper cousins. And engineering snipers offer little in either pvp or pve.
Arsenal mercs are more ineffective than some of these (madness assassin/lethality ops/ vig knights). But all these ACs suck in pvp to an extent compared to other ACs of the same class. That arsenal's 1% worse to take into a warzone than a lightning sorc is far less of an issue than that the spec that is
"good" in pvp is the worst such dps spec of any AC by a mile.