That would be really useful for PvE, but PI destroys in PvP and that's why it was nerfed. If you can't get a kill with three fusion rifle shots then you probobly arent cut out for the PI.
It's a special, but it has a perk that increases the drop rate of special ammo, and with each ammo pick up you get 15 shots which is 5 rounds.
But another little perk that it doesn't tell you about is that when it's firing in full auto it can actually shoot more than 3 shots. The extra shots don't even take away extra ammo. I've gotten up to 6 shots at once, and even was able to shoot it once when I ran out of ammo.