XBMC displays my content in a beautiful, highly customisable interface, whereas at best the XBONE will only show me a list, unless I buy content direct from Microsoft. A lot of content isn't available in my region when it airs in the States, so I get it through other means. HDMI passthrough will allow me to plug my XBMC media center into the XBONE and be able to use XBMC via the console (keeping the XBONE overlay over the top, for example), but that won't allow me to navigate the XBMC menu unless I use my flirc remote already plugged into the media center. But with IR Blaster it would be possible to send IR signals to flirc with hand gestures and voice control, in theory.
Hoping there are others that find that an exciting prospect :)