Forum Moderators: phranque
However, if you've got buggy code that doesn't validate, chances are pretty good that you're not meeting the needs of all of your users ... and if that's the case, then yes, you're harming your conversion rate.
Clean code will go a long way in helping your site to run smoothly and efficiently, on multiple browsers. A lot of people (me, included) need to validate our code to make sure it's clean and bug-free. It's a quick way to make sure that you don't have bigtime problems that you might not otherwise know about that could be costing you customers.