Take the forums for example ... when someone comes in with a question about their site not working properly in certain browser you say ... "Alright, let's have a look at the code" So you go look at the code, maybe spenda few minutes or alot more trying to find what's wrong. CSS is fine and the element that calls the css is fine as well ... so ... you sit there cracking your brain about this silly little problem when all the while, you forgot to close a div! Ah!

Now ... tell me: is validation is a waste of time?

If this person would have validated the site in the first place, it would've fixed his problem and saved ALOT of headaches.

Not saying the validator fixes all the problems of course but at least you've weeded out "potential" problems.