And, don't forget that with multichannel you get a center (as well as the surrounds). A discrete center. All playback, even that recorded with only two channels, sounds better with a center (derived). This has been known since the earliest days of stereo.
i'm not so sure. just like how we now hear much more information from Lps cut in the 50's and 60's due to dramatically better gear, the potential performance of 2-channel stereo is now much better due to better acoustics and gear. tests made in the 40's and 50's comparing 2 and 3 channels don't 'necessarily' still hold up.
when a room is completely purpose designed for 2-channel playback and the system is optimized for 2-channel i'm not convinced that a center channel adds as much as it detracts. i would agree that in a typical room that a center channel likely is a benefit if the music was recorded in 3-channel.
when i added multi-channel to my 2-channel room there were things i liked about it; but a year later i removed it because ultimately it did not satisfy for music to the degree that i looked forward to listening to multi-channel. i do have a completely separate Home Theatre 7.1 system in a separate room with front projector and all that for film.
the best of my 2-channel software (Lps and RTR tapes) out 'multi-channel' my multi-channel in terms of musical involvement. more real space, things are where they should be, and there is more ease and naturalness. of course; my room is optimized for 2-channel listening in terms of diffusion and live-ness.
i used a phantom center in my multi-channel music setup. however; i have been involved with comparing a center channel to a phantom center and personally i prefer the phantom center (using the 3-channel RCA SACD's) for music. for film i prefer a center channel for dialog.