Why does it matter? Not all browsers support the proper mime type to serve an XHTML1.1 Strict page anyways.
What benefits? I see nothing wrong with serving an xhtml doc as text/html. Everyone does it, no one complains.
Why would I cripple my site for over 70% of my viewers?
Yeah, it really is extra work.I make my sites XHTML Transitional most of the time, and present it as text/html. I have had no issues with this at all. Maybe it's not proper. And maybe it pisses of some people, but the site works, renders exactly how I want it, and no one got hurt because of it. I see no problem there.
IE is still the most "popular" browser. Even though everyone I know, for the most part, uses something else, the reports I get for my sites tells me that IE still dominates over everything else save for a few, where it is still at around 70%.
Again though, what is the benefit of presenting a true XHTML document?
Personally I prefer the markup xhtml forces me to use over regular html. It feels tighter and cleaner to me, so I will continue to use it.
After all, running Windows without a decent anti-virus is like walking through a Red Light District after eating five metric tonnes of Viagra.