I use it extensively as my hand coding occasionally types letters in places they shouldn't be.
My justification is at a lower level than ranking: eliminating the potential for bot crawl errors in broken coding. If they can't crawl it you'll never get a chance to rank.
Code bloat has been cited in other threads here as relating to page speed which Google has said is now one of the many factors they take into account in determining ranking. People back away (bounce rate) from slow pages too.
Validated code might not help ranking but broken non-valid code can stop parts of pages being indexed, can cause links to vanish from the visible page and therefore never be clicked and can stop parts of your content appearing in some browsers. Having got the visitor to your site, you don't want to present a broken page.