It's not useless. It does tend to have lower single target damage than the other two though, but it's not that bad.
You're right, my use of the word useless was a hyperbole. The spec is awful compared to Marksman/Sharpshooter and Lethality/DF because it has not been able to put even close to the same numbers on the board in PvE Endgame. If you're a Sniper/Gunslinger and you're not close to the top then you're doing something wrong. I will say though that the spec does offer the highest energy regeneration rate out of any of them.
Incendiary grenade is an 18 second stationary dot, bosses usually move around. There's also a lot of lost points to defensive stats. It does offer more survivability, but if you can do the mechanics at least halfway right in most of these operations then you don't need it.