For me, it depends on job role. For DPS, normally I put on R the main rotation combo, while on L the extended combo. For instance, SMN I put Bahamut/Phoenix related spells on R while on L there are the primals phase spells. Hot Bar 1 is single-target, Hot Bar 2 is AoE. Buffs, CDs and situational spells on expanded. Same logic when playing Physical DPS.

That changes when playing healer. Heals on right and x-right, DPS on left and x-left.