Semantic XHTML code is good for all...
-> search engines, PDA browsers, screen readers (more accessible). But valid xhtml is
not equal semantic. Tables should be used to present tabular data only, not to build layout's construction. Other thing is that XHTML is a way from html to xml. So in the future we will build layout with own DTD(s) and more.
Have a look at
a web standards checklist.