With all due respect, the biggest issue with stereo vs. mono is not being discussed at all here: phase cancellation. In several forums I have been reading about people trying to figure out why their keyboards sound so good in headphones but then they sound thin and terrible when amplified. It turns out that when summed to mono some keyboards literally phase themselves to death.

This is no longer a matter of theory to me: one place I play has a stereo house PA and recently when I set up there I failed to notice that someone had switched it so that it was set for mono - immediately I thought my Yamaha 9000 Pro keyboard had a hardware failure... the piano sound was thin and missing a lot of midrange with some notes almost dropping out altogether. No amount of EQ could solve the problem. Once I discovered the switch my normal keyboard sound returned instantly.

This probably isn't an issue for people with mono instruments in the first place, like the singing guitarists I see using the Bose PAS in many venues here... in fact, I think the Bose system is ideal for them. But for stereo keyboards it isn't just a matter of whether half of the Leslie effect is missing or whether the audience is hearing you in stereo but whether your sound is cancelling itself out. It's a more serious issue than you might think.