Thorn is (aside from Hawkmoon, which is a PS exclusive right now) the most solid Hand Cannon in the game right now in PvP, and the same argument can be made for it in PvE. In PvP the DoT allows you to two-shot someone if you're getting headshots. In PvE, the DoT can finish off trash mobs that you missed the headshot on, saving you a bullet, it penetrates, making clusters of mobs laughably easy to kill, and the DoT can prevent enemy shields from regening. It's also a 331 handcannon, meaning you're going to be dispatching everything but the toughest enemies quickly.