The ideas in my post are barely mentioned in the last paragraph of your post: that's why I wrote it. I think you underestimate the extent to which being able to speak and write in standard English is important. That's what pg implied in his original post, and he's right. Maybe standard English is a "fashion," as you put it, but that's not very useful: what's more useful is to note that being able to master this "fashion" is important in terms of communicating effectively.
I'd say it's no more important than, say, mastering the knowledge that showering before a job interview is useful. But because it's language people feel inclined to state the obvious.
