I don't correct people's grammar or spelling, but it makes me wince when I see it, and much less likely to read the post.
If someone's taken the time to write out a thorough post, their spelling is correct (or one of the common mistakes, or an Americanized version of a word) and their use of grammar is correct, if not flawless (as an English Lit student, I run up against obscure grammar rules all the time which make very little sense, so I don't begrudge people who don't know them) then I am very likely to read it.
If someone has a long post, but the grammar is awful, the spelling inconsistent and the syntax clunky, I may read it, but will probably skip the parts with the worst mistakes.
If someone has written a short post, but everything is correct, I am more likely to read it than a long post with bad grammar and spelling.
If someone has written a short post, bad grammar and spelling, I will usually ignore it as not worth my time to decode the mess of linguistics and read, which is a shame because some people I know make good posts, but then in the excitement of posting mess up.
I think everyone should make the effort to use proper English, both grammatically and in regards to spelling, if not only to improve the quality of this forum, then also simply for the benefit of other readers. You must always, in a public venue of any kind, remember you have an audience, and act for them, not yourself.