Brian Wilson mixed to Mono because Phil Spector did (Spector was his role model and idol), but also because he was and is for all intents and purposes deaf in one ear (his dad when displeased with them liked to smack the three boys upside the head). For some reason, the Surfer Girl album was offered in true Stereo, the only one that can be said of until 1968’s Friends, which Brian did not produce. Avoid at all costs the Capitol Records LP's labeled "Duophonic". That was the name they gave their 'Electronically Reprocessed For Stereo" mono recordings. Dreadful!
Spector mixed to Mono because his market was singles buyers, and 45’s and all radio was Mono in the 50’s and 60’s.
The Beatles and George Martin spent a long time on the Mono mixes of their albums, leaving the Stereo mixes for an assistant engineer to knock out in a day.
The very early Dylan albums sound ridiculous in Stereo: his voice in the center, his guitar all the way to one side, harmonica the opposite side. Mono is the MUCH better way to hear those albums.
There is a group of "Back To Mono" proponents in the Garage music scene (the lead singer/songwriter/organist of The Lyres---a great group out of Boston---calls himself Monoman ;-), feeling Rock ’n’ Roll should be heard as a conglomerate mass of sound, all instruments and vocals blended into a single entity to assault one’s senses. Unless you are close enough to the stage to hear the sound coming out of each guitar and bass amp and the drumset, that is how one hears sound from a PA system.