Matt Cutts explains that you shouldn’t need to re-write your comments if they are poorly written because it shouldn’t affect the quality. First, this implies that proper grammar IS a quality indicator (So make sure that you aren’t using grammar that Google does not recognize) and second, it also means that Google can differentiate “main content” and “comments” on a page.
We have long theorized that pages that receive a constant stream of comments seem to rank higher in Google, so perhaps they are monitoring how many comments each page receives.
I personally believe that if you have 2-3 comments that are poorly written and you have a well written 800 word article, it won’t matter. However, if you have a well written 700 word article but 3000 words worth of poorly written comments (100-200 comments), then you might have issues.
Google returns results based on the information inside comments (how many times have I searched for natural cures and found the result I was looking for inside someone’s comment) That means they are indexing and counting the content inside comments as part of your page.
Bottom line: A few poorly written comments won’t hurt you… but if you want to rank as well as you can, make sure that ALL the grammar on the page is as good as it can be. Matt Cutts is there to make sure you don’t do anything bad. He is NOT there to make sure you get #1 rankings on Google. (That’s our job)