That's only one way to look at it though, and probably not the most relevant one, as people can easily level (and gear) 2 jobs that belong to the same role. This is especially true for physical ranged dps, since they mostly share the same BiS. In this sense, chosing to main and invest tomestones on a melee job is way more risky. A DRG cannot suddenly switch to MNK or NIN when it finally finds a party looking for a melee. So why should casters and ph. ranged be able to switch between jobs in their respective role end join any party and still be good? That would be way too convenient.