Ear training is really handy if you're playing with people that have trouble describing what they're going to do.
No theory guy - "Hey, do you know Song X?"
Theory guy - "What key is it in, and I can probably fake it."
NTG - "Key? I just play from my gut!"
At that point, TG can have NTG play a few bars, and ear training will help figuring out what he's doing.
This scenario, more than any other thing, is why everyone should have at least a bit of theory behind them. It gives us a common lingo in which to communicate.
Someday we'll win this thing...