To me the worst sound in mono are string sections. Beautiful, realistic string samples lose everything if they are amplified in mono. Yamaha Motif acoustic guitars excel because they have stereo samples too.
I think in medium rooms most listeners do perceive a certain degree of the stereo effect. But it depends a lot on the type of music played, in my case, e.g. when emulating a symphony orchestral sounds, it is very important. For dance music, it's surely less important.