Many Web designers and developers have been building Web pages for years. Just one person can create so many sites with so many pages that the thought of trying to convert all of them to anything is completely overwhelming. But if you've been building pages with an editor or by hand, you probably have pages that are not valid XHTML, and use old HTML styles like the font tag instead of Cascading Style Sheets.
There are valid reasons for leaving your site in an older state than XHTML+CSS, but make sure that you've decided to do that. Don't just leave your site old because you can't be bothered to look at it. And, no, I won't accept that your editor does it that way as an excuse.
XHTML+CSS is the latest iteration of Web design. It allows you to place your content into your documents separate from how that content should look. XHTML 1.0 is a backwards compatible version of HTML 4.0 converted into XML. This means that it is more flexible and ready to work with databases, dynamic content, and future developments in the field. XHTML is easy to learn, especially for designers who already know HTML. And Cascading Style Sheets (CSS) give you even more flexibility in designing the look of pages. CSS is also easy to learn. XHTML and CSS combined make up XHTML+CSS.
(ADVISOR EDIT: please keep links to your signature, thanks)