Usually, the mobs in encounters silently engage you, like soulless robots. But a psyched raider or a sadistic merc surely wouldn't pass a opportunity to throw a taunt at his presumed weak victim, before going in for a kill.
So, why shouldn't they shout something at the PC, like "i'm gonna kick your ass", "time to die!", "look, a pajama", "meat!" or something like this?
Maybe given the opportunity, the player might want to write his own taunts in a file which the mobs will shout - this will surely add some colour to the PvE.
What do you think? Oh, and feel free to leave your taunt line suggestions.