Just to re-iterate what I've heard here.....
Sub-Groups add noise by introducing a mixing and amplification stage, and each stage adds noise and coloring to the sound. If you are not "golden-eared" or in a venue with pristine conditions you probably won't notice in a complex mix.
On analog desks I have used sub-groups to provide a dynamics choke-point. Vocals, Bass, Drums, Pastor, Keys, Guitars each in their own group with their own compressor / limiter can keep things from running away.
Riding faders is mixing as far as I'm concerned. As a FOH operator you have the final say on what people hear. It is rare to find a group of musicians who so perfectly balance themselves throughout a song, that adjustments are unnecessary. Compressors really help out, one of the reasons I prefer digital to analog consoles. compressors / dynamics on every channel, no need for a common dynamics choke point, digital grouping, everybody routed to the mains in one big happy mix.
I've seen hundreds of FOH console operators and they usually fall into two camps. The mixers, who are actively part of the music, providing balance and flow through a song, and the Showtime Rotisserie ("Set it, and forget it!") engineers whose main goal is to make sure nothing is clipping and all faders are at zero. I am a mixer. I find that musicians who become FOH operators are mixers, while technicians with less musical training go the Showtime route.
I would rather train a musician on the mixer and technology than try to train a technician / engineer on how music should sound.
My 2 cents.