It doesn't seem wise to carelessly use the MP all the way to zero, especially if it results in a penalty like that. In fact, considering his circumstances (he's pretty much on his own all the time), it might be best for him to never drop it all the way to zero, unless it's a system where that's the way to gain a bigger mana pool. Right now there's the taxi driver demon still left, so technically the battle is not over, even if this particular fight is.