While many worry about wide spread stereo speakers' problems with audience perception close to one stack or another, it is fair to say that the majority of an audience are still in the 'sweet spot', and those THAT close to one stack or another aren't getting a balanced sound, even in mono, because of positional differences of the different speakers (unless you are using a VERY small PA) and subs - which usually take several feet to develop properly.

The one HUGE advantage of stereo rigs is the use of stereo reverb and effects, imparting a sense of three-dimensionality to what can feel closed in and artificial if done in mono. We don't need to pan drums and instruments to the corners - this will definitely impact those close to the stack, and I feel that any keyboard manufacturer should place width controls on pre-panned sounds and drumkits, but having the reverb go out to the corners, even if all the sounds are panned dead up the middle, imparts a sense of spaciousness and reality to an inherently artificial sound.