Good point MaizieD. The only argument I would attempt is that spoken English is a different being than written English. The latter should always aim to be correct; the former ... well, as long as people understand the point, then communication is always better in any form, than none.
I have a friend who teaches French and I used to support her in-class, using my schoolgirl French. I was worried about my French grammar (which was ... well, unique to me, would be the most accurate way to describe it) and she said, if you were to speak like that in France, people wouldn't mind as long as you were attempting to speak to them in their own language. A bit like when French people try to speak English I suppose.
I also think there is a big question mark over which is incorrect grammar and which is just a local dialect. (Coming from deepest Somerset, I know that I don't speak the way I write, especially when talking with local people.)
(I think I may have just started a sub-thread!