A "6" from me. The surround awesomeness just isn't here. Fidelity and cool extra features, yes.
Nice review. I actually don't mind the 'band in the front, overdubs in the back" mixing style of the later albums (the rears are kept pretty busy on denser songs like "Land Ho" and "L.A. Woman"), but like you I prefer the quad. What's weirder to me is how Botnick used the center channel.
Compare the quad and 5.1 mixes of "Love Her Madly"--the quad has a nice wide stereo spread of the drums in the rear channels, while the kit is reduced to near-mono in the center speaker with a slight spread to the fronts.
Interestingly enough, Botnick--along with Doors producer Paul Rothschild--is also responsible for the Best Of quad mix. I never heard that it was mixed specifically to impress label execs. but Jac Holzman was said to be one of quad's biggest proponents back in the '70s.