Starch, what I meant was that the overall mix is "stereo". That is, the lead vocal is in the middle (for example), a backup vocal is on the left, a lead guitar is on the right, and keyboards and so on are distributed around where there's space and [hopefully] they sound most natural. In this mix, each individual signal is mono so that it can take its place in the final conglomeration.
If you record that lead vocal in stereo, the end result (assuming you have equal signal on each side) is that you hear a mono vocal in the center. If you record that backup vocal in stereo, and pan the result left, you have a signal that seems to be coming from 2 different places. The result is that the listener's ability to locate that voice in the mix is diminished. You have at least devoted more hard drive space to that data (as a stereo track) than it needed (as a mono track).
I had never particularly thought about this until a couple of years ago when a local bluegrass guy who records at his home complained that "the better his stereo micing technique got on individual instruments, the worse his mixes sounded".
I may have the details wrong but it's certainly an effect you can hear.