I agree that on a casual forum the rules of grammar are, and should be, relaxed. But as you mention, this is a very diverse forum and folks whose native language isn't English would most likely find proper grammar much easier to comprehend. I know proper grammar makes writing easier to understand to a native English speaker. I completely agree that correcting grammar and spelling is boorish behavior and wouldn't condone or participate in it.
I will add my opinion about art, literature and writing. Most great artists are highly competent in their crafts. Jackson Pollack, who is most remembered as the painter who dribbled or dripped paint on the canvas was fully capable of painting in a realistic manner. He only begin truly expressing himself AFTER he learned the rules of painting.
And while there are some artists who never had to go through the painstaking task of learning how to do it "the right way" before branching off, (Van Gogh comes to mind) they are greatly outnumbered by the folks who weren't that fortunate.
So that stuffy and boring writing you find so off-putting makes the fun and creative writing possible. ya dig? There are very few worthwhile things that come easy.
It is akin to someone new to growing that says gardening is fun and creative (both true) so they have no need to learn about lighting, nutrients, phases of growth, etc.. They might get lucky but chances are they will fail miserably.
You may feel no one has the right to pass judgement on you but they do judge you just the same.