I don't really pay that much attention to grammar, although I think my grammar is reasonably good, for the most part. Of course, the problem can be that if you use poor grammar in informal situations, it's more difficult to use proper grammar when it counts. You have to think about how you're speaking... or writing... because you're not used to doing so using correct grammar.

Personally, I'd say your mom's bf is either showing off or he was toilet trained with a shotgun! What you describe is, to my way of thinking just rude. On the other hand it is certainly true, I would say, that people judge other people to some extent by their use of language (both spoken & written.) So if one wants to get ahead in life, using good grammar is certainly a plus.