How is an XBMC twice as effective as an HTPC? I don't see what it can do that an HTPC can't, besides play Xbox games (and the PC can play most of those, with better graphics). I'd actually argue the opposite; that the HTPC is far more effective than an XBMC setup. Plays any audio or video format I've came across, upscales to 1080p and sharpens video (ffdshow filter), plays new games, can emulate all the old game systems, plus you have all the obvious PC features (web browsing, e-mail, office stuff) which are all great on an HDTV as well. Only downside is it could cost more, depending on how crazy you get with the hardware. What am I missing?
I will agree with the point you seem to be making about HD support on the PC; from what I've heard it still isn't very well supported, and it bugs me that you need to buy expensive software after you bought an expensive Bluray drive just to play the movies; for what you pay between the drive and software ($200-ish) you could buy a standalone Bluray player and not have to worry about HD DRM stuff (HDCP, right?).