After the wave of standardisation and CSS-loving we've had over the past two years, I feel the tide is starting to turn back a little. Less people are conforming to valid HTML. Some are falling back to HTML 4.1, which is fine, but I'd like to make a stand for XHTML 1.0 from a different point of view:
Developers.
When you've got a team of developers working on say, C# code, there is a naming convention for everything. Camel case, pascal case, whatever, but it is good practise to enforce a standard. And that's what XHTML does - enforces a standard. Lower case tag names, quoted attribute values, what-not. It allows a development team to conform to a standard very easily, and I think that is an important benefit of valid XHTML in a team environment.