Why would you ever use Dragonite against Fairy?
The most valid reason for Steel Wing was prior to the gym update as a counter to Ice, and even then most preferred the dragon-typed fast attack. I'm keeping mine in case something happens along the road, but Dragonites really aren't that interesting any longer.
There is always a utility Pokémon that will outperform generic damage. In this case my vote is on Scizor.
EDIT: I just noted that there was a "defending" subtext, which makes the example even less thought through. Surely everyone lucky enough to encounter a poor Dragonite in a gym would still use Ice to tear it apart.