While minimally useful in Fallout and Fallout 2, Mutate! is a reasonable perk in Fallout Tactics, where it can be used to remove unwanted traits from NPC squad members.
In Fallout Tactics, Mutate! can be used to give NPC party members a trait when they have none already.
Behind the scenes
The image for this perk is a reference to Marvel Comics' Dr. Bruce Banner/The Hulk. The image of the split personality always appeared at the end of the introscene during the television series, which ran from 1978 until 1982.