The title says it all. How much do you think that bringing a site up to W3C compliance can really help? The official validation tool is at http://validator.w3.org/
I'm no expert on this, but I'd guess it's less about having complete validation, and more about having sites which the spiders, which browse the pages as if they were a text based browser, can read and navigate through. And the closer you are to validation, the better they'll do that. However, and it's a big however - there's plenty of examples of horribly coded sites that wouldn't validate in a million years and rank very well, largely because the content is super relevant and people keep coming back to them. If you have a very popular site than everyone links to and visits, you're going to end up ranking fairly well, because it's clearly relevant to someone or something. My personal feelings on this - I make my sites as browser friendly as possible. I'll keep validating up to (and sometimes after) completion. It helps you iron out any simple errors in the code (particularly if a lot of it is generated dynamically), unclosed tags, bits you've forgotten. If I then add bits of JS or whatever that don't or won't validate, I won't worry too much because generally, I know the site is ok. Most of my sites are based on a template anyway, and I'll populate them using PHP and a DB. If the template validates I'm halfway there. I'll also usually check the site with styles turned off. If it reads ok, I'm happy with it. That help?
It's easier to crawl when websites are correctly coded. Not to mention being able to place that badge on your website can increase sales. People like badges even they don't know what they mean.
Its always worth making a site w3c complaint for SEO purposes, it will enable the search engine to crawl the site easier.