Phase cancellation is the reason Bose does not recommend using a pair of PAS with a stereo keyboard. Apart from the problem described by Esh, the half-stereo signals emanating from a stereo PA will interfere causing some relative dead spots in the audience, depending on room acoustics, e.g., the bouncing described by Al. Perhaps, the stereo PA system that Esh dealt with was set to mono to avoid these dead spots with the unfortunate side-effect of causing phase cancellation as stereo signals were mixed to mono.

Pick your poison:
Mono--No spatiality and no stereo effects, but no phase cancellation ("everyone hears the same thing.
Stereo--Spatiality and stereo effects, but phase cancellation (people hear different things, dead spots).
Multi-mono--Spatiality, no phase cancellation, but limmited stereo effects.