True, on the other hand, including a doctype still makes sense to avoid quirks mode. (There'd be less errors if he'd use html5 doctype though.) And as long as it renders properly in A-grade browsers you're good to go.

That's true right up until it isn't, i.e. when you're struggling to understand why your javascript is producing subtly buggy behaviour.

I hate to be that guy but: http://en.wikipedia.org/wiki/List_of_Latin_phrases:_I#id_est

I know what i.e. means. The benefit to valid markup is specifically for ensuring that the javascript you write to manipulate the DOM works properly.