Part of the argument for designing with web standards (i.e. using semantically appropriate tags) is that it makes things more maintainable. But this only applies if you're coding in (X)HTML. For this site, pg codes almost exclusively in Arc so he only has to worry about keeping that code maintainable. In general, you should only worry about the source language, not the object code it spits out.

The other part of the argument relates to accessibility and to my knowledge the latest accessibility tech can handle "messy markup" just fine.