I tried that some time ago with 13.10 & xface-desktop and I ended up with some weird mix of the two (randomly some parts from unity and some fonts and menus + shutdwon splash screen definitely from xface). And I was not able to totally remove the xface packages from unity again. So I'm quite reluctant now :).
Granted, that concern is valid. Just let me note that it is normally not that difficult to reconfigure unity to use its own designs everywhere, but I think as well that it is not particularly nice that this is necessary.
