Because if someone is going to use a spell/skill/whatever against you in an aggressive way, you should have a means to counter that "tactic". Was an assassin casting cure critical on an enemy to raise him to 100% health so that the assassin could assassinate an "exploit"? No, but it was an unintended use for a spell that re-balanced the class and resulted in cure critical and staves being removed.

The point is, allow the invoker continue to cast fly in the middle of combat if you want to keep the tactic in the game and give the invoker that kind of power, but at least give the victim a saving throw against a spell he absolutely does not want cast on him. In other words, make it a trust spell like metabolic slowing.