well, I guess some grammar is crucial to intelligibility, not necessarily the prescriptive grammar. When a lot of people make the same mistakes, they share a new code, new intelligibility expectations, a new grammar.

I think many people forget that when talking about how the newest generation mistreat their language, or whether foreigner's mistakes can be understood.

heck, sometimes it's funny when a native speaker tells me that I speak X very well (I don't), just because I use some prescriptive form correctly.