My monitor setup was stereo when I used FRFR speakers, now I use in-ears and they remain stereo.
My band's our PA system is also stereo, and we always run stereo to any clubs with an in-house PA as well. Makes a big difference, and most people aren't parked in front of one set of mains or the other, so most people get the full effect, especially when I have things set "wide"!
I have been running stereo since the summer of 1991, pre-FractalAudio (I was using ADA at the time, with the split stack!) Back then, there were 2 guitarists, and I was panned L+R in stereo and the other right up the middle, since he didn't use many effects.
Now I am in a Guitar/Bass/Drums/Vocalist band (3 of us sing), so I have a lot more to fill in, and our sound man says the stereo sounds wonderful in this context, especially when I am being overtly stereo effects heavy in some sections.