I think my favorite thing about the CMs is getting to pick three Umas that fit the track out of my favorites.
The multiple uma setup and the little blurbs before and after each race let me play out the 1 trainer to multiple umas fantasy like the anime in a way that Team Trials just doesn't really work well for, because with 15 Umas it's hard to seriously self insert as a trainer for every single one of them or keep track of what the dynamics between them would be.
With just 3, it's a lot more manageable, and I even get to make a little celebratory concert with them as the three leads after the cup is done, and I can make them three of my title screen umas (with the remaining slot being for whichever one I'm in the middle of a run with). It adds a lot of fun to playing the game!