If there's so much air getting in that the pump doesn't get cooled and thus wears out/burns out.
If the pump happens to have a clam or sponge or squirt in its path that gets enough air blown in to cause tissue death or complete death. Probably not a big concern. And if you have a clam directly in the path of a powerhead it'll probably die anyway.
You get air bubbles naturally in the reefs as the waves crash, so it's no big deal. It's a little more natural, but like Jeremy said, it's an eyesore...just like algae.