mhansen - 2:19 pm on Feb 16, 2011 (gmt 0)
I look at valid code the same way I do page load times, being slow-loading or dirty-coded won't really hurt you too bad in serp's if you are the authority on the topic... but being fast-loading and code-tight may definitely help your cause!
In other words... I cannot think of a single reason why anyone would NOT want to validate code. Its a few minutes to a few hours of time, that will have no negative effect.