What I'm thinking is....
Your tracks end up being played through mono rigs.
All the stereo spacing achieved by widening is lost when the signal is converted into mono and driven out a mono rig.
So if your mix is balanced using widening rather than eq balancing the two instruments sharing the same frequency range which are seperated through widening will end up occupying the same space once your track gets played out in mono, and this could lead to a muddy cluttered mix.
So whilst widening gives things room to breath it only works if you hear it in stereo, and the moment you revert back to mono your mix ends up sounding cluttered.
This is my mistrust of stereo - I don't want to create the "illusion" of space only to find that my mix ends up sounding muddy when run in mono out of a club PA system.