I, for one, am into life-long learning, regardless of the form that takes. When we are teaching our students the rules of grammar and the like, of course we should adhere to standard, international English. However, having a conversation on the message boards is another story. I enjoy learning new phrases and idioms from other cultures. I'm from Canada, and I totally surprised a friend of mine from the UK when I declared an attractive woman to be "quite fit." He was impressed I used it properly in context. I love incorporating new things into my speech!
That said, I think teaching some "local flavour" or idioms to your high-level classes is never a bad thing, either. Just make it clear that it is a special case, and not necessarily "proper English." I would remember to keep that confined to your high-performing students, though.