Animation is suitable in an interface, an icon does not constitute an interface. An icon on a button animating on rollover is just distracting. Now, perhaps if

Animation is suitable in an interface, an icon does not constitute an interface. An icon on a button animating on rollover is just distracting. Now, perhaps if something had to go on in the background it would be relevant (indicating that the work is still in progress) but that's a very edge case.