As a member of the SE Community Team, I can tell you we consider basic grammar and punctuation to be extremely important. I encourage you to keep your standards high.
The SE network has some of the best information and most productive discourse anywhere on the web, and adherence to proper grammar and spelling are one of the ways we keep things orderly.
I think attention to detail and clarity is important when you're asking the world to help you for free.
-- Sean Owen
This is exactly it. A post full of grammatical errors is harder to read, which in turn makes it harder for people to help you. It becomes a matter of courtesy and mutual respect, and like our moms all told us, you need give to respect to get it. If that equanimity breaks down its very hard for a community to remain productive and valuable.
Does that mean there should be zero tolerance for spelling errors, or that we shouldn't be welcoming to users who aren't native English speakers?
No. If you're in that category, we want you here too! Instead, I'm recommending developing well ingrained habits toward editing those posts to improve them. Doing so gives those users a hand up.
The information on this site is intended to be around for a long time, so it's wise to ensure it reflects well on everyone involved.