Well yes, if you are going to end up playing it through a MONO PA, you might as well use MONO from the beginning.

However, as touched on, the stereo field adds a spacious depth to resemble more of the visual setting you mentioned. Rhythm guitar is on the left, lead on the right, etc. to help distinguish them like you would see them visually. When I do a mix, that's one way I judge it; does it sound like how a band would set up and perform it?

To reverse your logic about sound spread across a sound stage .. MONO causes things to sound like the band is all standing in the exact same spot.

I do not work here, but the benefits are still awesome
Make your sound your own!