If we had what the OP suggests, it'd only lead to situations like;
"looking for Tank, must be lvl 50 Warrior, Paladin, Monk and Dragoon with a minimum 35 WHM"

While i absolutely would love to be able to tweak the way i play my Monk, you'd only end up with 'cookiecutter' theorycrafted/SIM'd Job combinations that the community declares you simply MUST have in order to be in their groups. Oh, don't have the right class leveled? SCRUB get out of my party!