I think grammar and spelling are important, but I realize this also gets better with time. I have read some truly excellent hubs that have had typos here and there. That didn't stop their content from being excellent or keep me from wanting to read it.
If a hub has multiple typos or is written in "text speak" or something then yes, that is a turnoff. Also, you have to keep in mind there are writers on HP from all over the world. Many ESL writers make more mistakes in grammar and word choice.
I think it's important to proofread and do the best we can, but we all make mistakes. I think people who nitpick errors tend to be insecure in their own writing strength.