Batrider has flames concept on the default Warcraft III, you know. It has liquid fire and unstable concoction (even if the second seemingly isn't of a fire concept, it does make the unit explode and burst into flames).
Plus, fire would give that "extra" feeling for the ability. An ability that produces "extra" sight (or whatever the concept would be), it would require some enhanced graphics for the icon, rather than just the bat's plain eye.