Since stereo sounds so good on recorded music - we wouldn't THINK about not creating a stereo mix for recordings - it seems like we should also try for stereo when playing live, right? Well that may or may not be true. But if you think about it, recorded music is almost always listened to when were are close to, and between both speakers - in headphones, over car speakers, on docking stations, home entertainment centers, etc. But at a live concert, there will be folks NOT between the speakers at all, so they might only be able to hear the stuff mixed into one of the L/R channels (not good).
Here's an article that talks about the pros and cons of trying to use stereo for live sound.