True, but most of a Beholder's own offenses also don't work within the confines of its antimagic cone, so it's not like it'll be facing that towards you if it wants to kill you.  Though now that I think about it, standard Beholders also don't really have access to much in the way of force effects or other things that specifically target incorporeal beings, either.  I suppose it would have to be a Beholder Mage, then...