About 6-7 months ago, I began validating the web pages at work as we made changes to them. We did some heavy work on cleaning up all of the errors and now all pages when they go live are valid. And so, I have started to see the benefit of validation. More browsers are happy :). We have seemed to get a better seo ranking on some of the terms we wanted to move up in. And the coders now find it easier to edit web pages.
So all in all, it has been well worth the few extra minutes spent up-front to make sure the pages are valid.
Now we are working on making HTMLCenter.com compliant with the 4.01 spec. About 35% as of this posting. Most of the issues are not in the template as those are now fixed, but with updating the hundreds of content pages across the site. It is actually fun cleaning up the pages as I have learned many new things about how HTML works.