I do not think that it makes much sense to merge this with XHTML, as the integration is a separate language profile. By way of comparison, MathML merits its own page, although it is generally embedded into XHTML.
The SMIL integration with XHTML does build upon XHTML, but it supports functionality for multimedia and so does not have the same audience as XHTML support does. A link from that page makes sense, but a merge does not. You can also take a look at the W3C site and note that the XHTML+SMIL work is mentioned in the HTML working group area, but is documented in the SYMM (synchronized multimedia) working group pages. [Should have been marked as editing by cogit - I was an editor on the associated W3C specs]