Why would you want to use XHTML?

Normally, you might upgrade to a new version of a technology for new functions, or because problems with the previous version have been fixed. However, XHTML is a fairly faithful copy of HTML 4, as far as tag functionalities go, so don't expect any fancy new tags.

The W3C states that the primary advantages of XHTML are extensibility and portability:

Extensibility

XML documents are required to be well-formed (with elements nested properly). With HTML, the addition of a new group of elements requires alteration of the entire DTD. In an XML-based DTD, a new set of elements simply needs to be internally consistent and well-formed to be added to an existing DTD. This greatly eases the development and integration of new collections of elements.

Portability

Non-desktop devices are being used more and more frequently to access Internet documents. In most cases, these devices do not have the computing power of a desktop computer and aren't designed to accommodate ill-formed HTML, as standard desktop browsers tend to do. In fact, if these non-desktop browsers do not receive well-formed markup (HTML or XHTML), they may simply fail to display the document.