Yes, sadly plasma weapons as is are pretty worthless. They're reasonably strong, but are largely TU inefficient, inaccurate, have a small clip size (Blaster aside), and are painfully short ranged. For all these reasons I too find lasers and even some conventional bullet based weaponry to be preferable to them. In all fairness though, lasers tend to be superior to most weapons, including particle beam weaponry due to their reliability save in cramped levels where more TU efficient but inaccurate weaponry is optimal.
I might consider using plasma weaponry in a niche CQC assault role if the damage were to be substantially increased.
EDIT: Looking through the most recent 2.3 stats, I find that the plasma weapons have been significantly improved. I'll have to do some more in game testing to determine if they're now truly viable. My only complaint is that their automatic (including burst) modes are horribly and blatantly TU inefficient as compared to those of their conventional counterparts.
