Validating a website has become standard practice among designers over the last few years. As a site nears completion, the designer tests the site against the W3C Validator service to ensure that the code meets the standards, allowing them to proudly display a badge declaring their competency.
I have no real problem with this, except insofar as it provides no value (except to the designer's ego!) to show this badge.
The real problem lies with the claim that validation is inherently valuable as an SEO tactic. Sites like Google, Amazon and a plethora of other high-ranking sites stand as proof positive that validation alone will not ensure your site ranks highly.
So why is validation touted as an SEO technique?
While Joomla's core article system is reasonably robust and the introduction of template overrides made it possible to more easily theme elements of Joomla that were previously untouchable, K2 takes both theming and editing to the next level.
Designers have much greater control of each 'view' of content presentation and editors have less to worry about when publishing articles.
It's a win-win situation for aesthetics and usability.