I've got a few of those Audio Fidelity recycled 4.0 quad CD's and, while not exactly musically accurate, are a lot of fun. Yeah, the center is not needed so they are perfect in this application. Granted, four identical speakers would be nice but it's not always practical. But, this is where having surrounds from the same manufacturer/ line comes in handy. it seems that doing away with the center channel doesn't hurt the experience a bit.
As for those Living Stereo SACD's, well, I think my mains are too close to really appreciate that center channel. Remember, these were made when stereo was in it's infancy and the mains were widely separated.