This used to be a thing with 1.2X speed runs. People would optimize the party structure for every part of the dungeon, from stacking BLMs for specific pulls, to very exact boss setups. I remember one run in Cutter's Cry where I must've switched class like 3-4 times, between BLM, DRG and MNK. BLM for trash, DRG for midboss, BLM for trash again, MNK for last boss if I remember correct.

Anyway, it can get out of hand because people will use it to optimize things and stop accepting people in roles where they can't handle 2-3 classes perfectly, or have them leveled and geared in the first place. If it was a forced "roulette" kind of thing in easy content, it might be fun.