Your joking aside, class balance should aim to have tanks performing as close to as possible to equal with other tanks, same thing for DPS, and same thing for healers. PLD, WAR, BST and SAM (adding the last two for good measure) should perform within the same parameters as they're all tanks. If WAR is better, or SAM has higher DPS, then you lead to things like the community taking the "best" of the four and kicking everyone else to the curb.
DPS is also notorious for suffering from this problem. If BLM outshines everyone, then everyone will just stack BLMs and kick everyone else to the curb.
As far as how this applies to RDM...I've mentioned it before, but RDM would be a lot better off if we had talent trees for the hybrids so that if you want to spam cures and refresh you can, if you want to front line and use your sword you can, and so on. We don't have that and never will because of the armoury system's restrictions. On a personal level, I therefore support the RDM's gameplay being built around the rapier with magic (attack and utility) supplementing that part of the design. The job can and should still have some form of heals and nukes (as the job has always been sword, white magic, black magic), but within a focused role that makes sense for the job's concept and aesthetic.
You can't do a strict 3-way split between sword, white magic, and black magic because that's how the first draft of XI's RDM was designed, and it had to be changed because few could find a use for the job. Sadly, XI's developers chose to go the support-bot/cure-bot route.