It appears that almost nobody except Microsoft themselves are publishing documents in XPS format. Not really surprising — why would anybody use it over PDF, for which readers are near-universally installed?
Searching Google for XPS documents vs PDF documents:
Well actually, XPS is by default (as I know it) installed on every computer with Windows Vista, Windows 7 or Office 2007 (or newer).
PDF has to be manually installed by downloading Adobe Reader or some other freeware equivalent.
I personally however prefer PDF to XPS as you have more choice, and making PDF files is easier than making an XPS file.
Stupidest question ever. That’s like asking ‘Why use Linux when Windows is near-universally installed?’
Uhhh… Well, no. If you’re publishing a file publicly, you’d be aiming for the file format with the biggest population of compatible readers. XPS readers may get there eventually with MS pushing it on their platforms, but it seems unlikely that the format would reach critical mass to take over from PDF any time soon.