W3C doesn't say that it's OK to serve XHTML 1.1 as text/html. It says that it's OK to serve XHTML 1.0 as text/html.
The thing to understand with HTML5 is that it turns the relationship between syntax and mime type on its head. Instead of saying which mime types can be used with HTML and XHTML, HTML5 says that if you serve a page as text/html then it is HTML, and if you serve it as application/xhtml+xml then it is XHTML.
So the doctype doesn't matter in determining whether the page is HTML or XHTML. This is what browsers have been effectively doing for years, though it should be noted that most validators and some authoring tools do make use of the doctype to determine modes of operation. The new HTML5 validators such as validator.nu do use the mime type as the determining factor. What authoring tools should do is currently unclear, and is still being argued about within the HTML working group, but it seems likely that they will have to fall back on a mapping of file extensions, (e.g. filename.html versus filename.xhtml)
If only because HTML5 validators and all browsers do the same thing when confronted with the HTML5 doctype, its use has to be a good thing, even if you're not using any other HTML5 functionality. However, you should note that not all valid HTML4 is valid HTML5, and that many details of HTML5 are still being argued over. In some cases, recent and proposed changes may not be implemented in the HTML5 validator, so if you check your pages you may get things reported as errors that are valid HTML4, and may eventually be valid HTML5.
Finally, note that when serving your page as application/xhtml+xml, the HTML5 doctype is completely optional. Browsers simply ignore it.