I have to think that while something like Plex would be better for a lot of people, XBMC still gets used on name recognition alone. If you have more than one device that you watch media on (TVs, Roku, tablets, phones, whatever) why wouldn't you want a central server managing the library, downloading metadata, saving watched flags, holding resume times, and serving up video to the devices? I turned a friend on to Plex from XBMC and he's amazed at how often he stops watching in one room and resumes in another. I love it too. I can't count the times that I've started watching something on the iPad in the kitchen while cleaning up and then going into the bedroom to finish on the TV. That's a way bigger feature to me than getting "the real deal" running everywhere I need it.
The people above wanting this for Google TV...check out Plex, it may be exactly what you're looking for.
Sorry to not gush for XBMC, I know it's the best solution for many people and I truly appreciate the heritage and the fact that it's the foundation for Plex, but until they have a centralized server (if ever), I can't even consider it for myself. And no I'm not going to jump through hoops to get it.