First of all the most important person is the player[Stereo is so nice, one of us should enjoy it].

There are places I play[with the band],that stereo will not work as well..primarily because we have to fill the large room up with coverage.

Most of the time Stereo works fine, especially on the dance floor[the second most important people]..as Clif mentioned, there is no substitute for spreading the "Band" instruments..
Usually I place the speakers[on poles] about 15-20 feet apart in stereo, even with 200 people[dancers] on the floor in front of the band..
The lone situation that comes to mind, We play a club that is wide[all dance floor], and I have to angle the speakers outward slightly for even coverage[mono is the only way to work this room..].