I think the point is -- there is no valid </br> tag for a reason -- it's not valid.

Also, one thing that no one seems to have mentioned at all in this thread yet, is: why should one use the br element at all, be it in HTML or XHTML or whatever? Granted, even I occasionally do, but that has really to be a rare thing - most often I don't want to barely break lines within paragraphs, but I'm fine with splitting the text across paragraphs, thus using <p> tags. Having a background (although not a terribly vast experience) in TeX/LaTeX, which are all about good typography, I feel compelled to remind that that's the same recommendation that's usually made there, although the markup is completely different. Paragraphs are plainly separated by empty lines (or explicit \par commands, but you generally don't need to) while the breaks must be inserted with \newline or \\: thus in that case perhaps the UI makes it slightly more convenient to shape the text in paragraphs, than to use returns a' la average quadratic dumb wordprocessor luser...