Defiance makes sense for 72-man content when you are pushing the enemies front lines making you their nearest target and thus getting lots of aggro. If you're not getting hit or focusing nodes or w/e it's safe to drop.

24-man content, I almost never use defiance since it's always better to have the added dps.

A warrior in pvp is best suited to pressure the opponents healers and Black mages. A BLM with a warrior all over them will be completely shut down. You don't necessarily have to kill them, having them blow their cooldowns on you or having the rest of their party focus you is exactly what you want.

Outside of that, a well-timed group heal, Limit Break, or holmgang can certainly save your group especially when getting double teamed.