Matt Cutts, master Google guy, brings up a good point. Although he is talking about general spell checking of your Web page content, it really goes beyond that.
The example that he shows has two mistakes (although he only noticed one initially). The word guarantee has a typo, and also don’t is missing the apostrophe before the t.
The point is simple: content counts. The example given is trying to sell you something. You can do all the SEO (Search Engine Optimization) you like, and bring in hundreds of thousands of eyeballs to your site, but if your content is filled with spelling, grammar, and punctuation mistakes, you will not be able to convert them to sales (or whatever your goal of the site is).