Seeing that many people who use xbmc use it for playing terribly outdated formats and containers like .avi, (and often from suspect sources) then I don't think quality is much of a concern for them.
I use modern high quality formats and containers like .m4v and h.264 but if for some reason I am forced to use something like .avi then Air Video + AirPlay works great. And the transcoding is done with bitrates at up to 5mbps which is good for anything up to 720p HD. And of course this method also gives you the option of switching the video back and forth between your mobile device and all your TVs at the tap of a button. If all you want to do is play unsupported formats I believe this is a more attractive option.
10 years ago 'many' people were using dirt cheap xboxs to run xbmc and play 'outdated formats and containers' in a way that still can't be matched by any other htpc solution.
I'm not sure how airplay @ 5mbps matches up to a htpc front end like xbmc
I don't use an atv, but a cheap ION box is a 'more attractive option' to me, and will easily do 50+mbps h264 and have all the advantages of a 'proper' OS. The only positives I see about switching to ARM and the ATV is a few watts of electricity and a more seamless extension of itunes.
Luckily I haven't been living in the dark ages for the last 10 years, don't touch itunes and expect far more out of my media consumption than many...