I play the 3/7/31 Sever Force spec and it is perfectly viable in both PvP and PvE. Played right, on a good day with a bit of luck I can take out any class in 1v1. Sure, I can't match the spike damage a Hybrid Sage can throw out but I have more survivability, more utility, just as much, if not more, mobility, and with correct use of your dots you can easily have 5 Weaken Minds, 2 Sever Forces, a PoM MC and a Force Quake or a TK Throw on the go at once, which is a hell of a lot of damage for a healer to deal with.
Not to mention the fact that a 2 second immobilise regardless of Resolve is a game saver in Huttball or when you're kiting or chasing someone and your DoTs, unless they are cleansed, can prevent a plant or a point cap regardless of a healer healing through them or not.