I recently battled a person online. It was a really tight battle with them having a slight edge over me. My Vaporeon was my only remaining Pokemon and their's was an Yveltal. Vaporeon had just under half health left and was paralyzed. I presumed that because they had the speed advantage and because I was paralyzed, that my chances of pulling off a win were next to zero. For some reason, they proceeded to use Fly against me. Now I understand that this would prevent me from hitting them with an Ice Beam and thus enable them to tie up the match at the final stage of this battle, but due to insane luck, Fly missed and I managed to hit with Ice Beam and win.
What I am saying is, apart from in this way, can Fly ever be useful competitively? I personally don't see it as a useful move apart from in-game.