That's a great question. Companions reviving with half-life is part of the balance of the game.
Pros: Companions revive with half-life, an unlimited amount of times.
Cons: Companions can't revive with enemies nearby, don't recover mana when they revive, and have a delay before reviving.
Cons: Companions often step on discovered traps, will often stand on fire tiles (and other damage tiles), often do other really weird AI things.
Cons: Companions get significantly reduced experience, get reduce number of skill and trait points at level up.
Cons: Companions have pre-selected Advanced Skills.