You don't know that this won't be the case. BRD and MCH have TP/MP/damage buff application, NIN has enmity manipulation and TP regen, and I can see RDM having some sort of support function in addition to its damage. I always envisioned RDM as kind of the caster equivilent to NIN, with refresh being the MP goad, and having a few abilities like haste/slow/dia to give the job more utility than straight damage.
