There are several reasons valid code and standards compliance are very important. Here are a few links that discuss it.

http://www.stuffandnonsense.co.uk/ar...ds_trifle.html
http://www.maxdesign.com.au/presenta...fits/index.htm
http://www.w3.org/QA/2002/04/Web-Quality
http://www.webstandards.org/learn/faq/#p3

As an instructor I don't allow my students to hand me invalid code, I have failed projects that looked great but were invalid non-standards compliant, non-accessibility complient projects.