What with life speeding up these days, workloads getting heavier, [IE] getting more popular, and people getting lazier, validating your pages can seem quite a chore - especially if you haven't written standards-compliant markup to begin with. You may be feeling, "why bother?"
Bluntly, in my not-so-humble opinion, this is a silly attitude. Validating ensures forwards-compatibility, removes any reliance on certain browsers, and (usually) allows good backwards-compatability as well. Besides, "this site compliant with W3C standards and best viewed with any browser" sounds a lot better than "go get IE6, you inconsiderate troglodyte".