Of course not, but you seemed to be saying that if any semantic meaning is desired, it should be encoded as such and made available separate from the HTML... I don't agree.

Take Microformats, for instance. Say you're already displaying Events somewhere on a site; to add Microformat support all you do is add a few attributes to your existing mark-up and any Microformat-aware browser can properly recognize it. Sure, you could create another file entirely that contains XHTML specifically created as a Microformat, but that would be redundant and not nearly as helpful to visitors.

Or, say, a sitemap. Many times it makes sense to have a completely separate XML-based Sitemap for, say, Google. Then again, what if you just added the requisite semantic mark-up to an existing sitemap, one your visitors can use, but so can Google? You basically get two for the price of one.

RSS is a different animal and in most cases couldn't logically be encoded into the XHTML itself because you wouldn't have every RSS item on one page, etc. That's why the <link> tag is used to include it in the source, just like you include Javascript and CSS and so forth.