Why should I use XHTML?

This site does not use XHTML although it will some time in the future. The next version of the IssueTrackerProduct will. I've never really understood why one should use XHTML but this site gives the answer:

"What's in it for me?
In the short term, probably not a lot, but the Web is moving very quickly and you always have to think ahead.
If using XHTML only helps to tighten-up your markup, getting rid of ambiguities and sloppy coding, then it's already improving the robustness of your pages across browsers."

and

"The use of valid XHTML helps to guarantee that any pages you build now will work well into the foreseeable future in all current and future browsers."

Good. Now I know.