Alter self for non humanoids is generally considered unbalanced, letting them do stuff humanoids can't like gain a fly speed is just one of the reasons. The spell was designed to be cast by humanoids.<br/><br/>But the entire polymorph line does impossible to properly balance things so its not just alter self.<br/><br/>At least it can only grant EX flight which is subject to not working if you go unconscious or are paralyzed causing you to plummet to the ground with a thud. Fly is saved from that indignity. Fly also grants "featherfall" for 1d6 rounds when it ends/is dispelled which can't be dispelled, another bonus over alter self.