People say "wanna" and "gonna". I think the important thing is to teach learners that they will hear them but they should try to use correct English when speaking and never use them in writing.
I agree with you when you say they should never use such expressions in writing, but I don't think there's anything wrong if a student of English as a Foreign Language said:
I got that habit off a friend of mine.
I got stick off my dad for coming in late.
I'd be chuffed if one day a student of mine came up in class with an informal statement like those above. It would probably mean they had a chance to use English with their friends outside a formal learning environment.