Why are you even comparing ranged dps to melee? Powertechs have great burst, merc survability is trash and their dps is low if they are focused, I agree with you, but still you shouldn't compare them.
So in comparison to other rangeds:
-sorcs are immortal, they are really hard to kill if they are skilled
-snipers have great burst, they can put out some nasty dps if they have good support (I mean real dps, not multitarget dots)
-mercs... they are here just for electro net