First off.
No Full auto Lasguns(without multiple barrels or the recycling mechanism of the multi-laser...which btw is too bulky and complicated to be used as a handheld weapon).
According to fluff it's semi-auto since it has to bleed off residual energy and reset.
Second.
The advantages of a short(short as in microsecond) more powerful beam is obvious.
1. You hit the same spot, more heating in the same spot.
2. There is only a very short time for the energy to dissipate though the target material so the blast is more powerful.
3. If you deliver all the energy at the same time you get an explosive combustion in the target instead of it just being vaporised. It's the difference between a trail of gunpowder on the ground and the same amount of gunpowder in a keg. You get an explosion which gives more damage than just a burnhole.
Third, Little known Fact: You do know that the energy discharged from a .50 BMG shell is sufficient to power a laser to blow a fist sized hole through a man. If we can tame chemical energy to that degree into electomagnetic energy, then we could have chemo-electric discharge batteries that are as small as a cellphone that is capable of powering an energy weapon with the power of todays assultrifles.
And finally. The lasgun isn't powerless. It's just that the weapons that it's measured against are way more powerful. I mean. the weapons that it's measured against are the 20mm handcannons of spacemarines and orks that fires minigrenades, the superadvanced shuriken catapults of the Eldar(mono-filament discs fired at supersonic speeds. Slice and dice) and the plasmabased weapons of the tau.
