While multich stereo modes do require more power, personally I only use them for parties, just don't like that mode for normal listening and if I am going to use multich play, I'll use a surround mode. I can't think of an avr that rates in ACD because that's just not going to impress as much as a 2ch rating....marketing wins. As was said, it could be 30%, it could be 70% (and the Denon/Marantz thing I believe is for 5ch driven only, not all the possible amp channels), could even be more, depends on unit. Just not a very important thing for consideration IMO. There are many who do like two or three stereo pairs for some reason, look at all those collecting "vintage" 2ch receivers and doing just that with the A&B settings.....kinda nutty to me but YMMV.
ps as far as 4ohm rated speakers, yeah an avr may not be the best tool for the job, but depends on your playback demands. Better to get better designed speakers for your gear.