Oh, I agree that it doesn't matter... You can get by without it.

Just as well as you can get by without a modern arranger, and a modern PA, and a decent voice. Plenty of players managing to do that. But would you WANT to?

Let's face it, few of us are really doing all we do because the audience hears it, or recognizes it's worth. We do it for the few that DO recognize it. And we do it for ourselves, because if WE are not happy, how can we make our audience believe we are?

Having grown up playing live music all my life, I find mono to be the most unnatural sound ever. You NEVER hear mono in the real world, there is always SOME kind of spacial cue and spread to the sound. Listening to mono while I play only accentuates for me the realization that I AM playing with a machine, and anything I can do to help dispel that impression makes me a happier player...

But I HAVE played through many mono PA's, and know perfectly well that you CAN do it, and even that few of your audience cares. But I try to play for the few that DO, and for myself. And other than carrying around two speakers (and even most of us that perform in mono do that), it involves no more equipment than being in mono. So.... why not?