Allowing a player to control the witch (an instant knock down) is extremely overpowered, especially in versus mode, as DeadHerald said.
50%+ of the time, in a full versus game, the infected team will spawn with 3 capture infected (charger, hunter, smoker, jockey) and one support infected (boomer, spitter).
So, if there is a witch spawned, the support infected can just run over to the witch and wait, while every other infected player is doing the same. This gives them four "capture" infected, and if they are coordinated enough (it really isn't hard to do this) they can get an instant win on the survivors.
EDIT: I just realized that this only matters for versus mode anyway. Oops.
SECOND EDIT: The infected players could also shamble the witch from her starting position if they go second and notice where the witch is placed, as close as they can toward the starting point so as to strike at them with that 4 full capture infected hit so as to minimize the maximum amount of points the survivors who go second can get.