There are just a couple potential downsides to ears.
One is that if a fly does get inside, they'll sometimes work their way up to the ears and get stuck in there, which is worse than having nothing covering them so at least the flies can come and go. I haven't seen this happen to my boy, but I've heard of it happening to others once in a while.
Two is that some horses (or at least one at my barn, lol!) knows how to grab the ears of other horses' fly masks to pull them off. Everyone in his field has earless masks right now, it's the only way to keep them on.
That said, I prefer ears on my boy. He gets annoyed when flies are in his ears. Like an earlier poster, I also always ride in a bonnet this time of year.