It's a very different issue here though. In WoW, if your class went completely out of favour to the point where you had to switch, you lost your name, your faction work, all your accumulated gear and your even reputation within the community to some degree. As things stand right now in FFXIV (As far as tanks and healers go), you lose your relic progress and that's all. The bulk of your gear and of course, everything else with your character stays unchanged.
This is something that FFXI played on well (I'd usually start raid evenings out as SCH and end up on either BRD, WHM or BLM depending on the content and the roster we had that night) and I'm a little surprised that FFXIV hasn't picked up on that.
In the grand scheme of things, does it really matter if one of the tank/healer jobs is viewed as non meta to the others for that tiers end game content? As long as it offers a sufficiently diverse play style and is genuinely viable it'll get used and appreciated, lets not forget that a BLM could heal a significant chunk of the content in this game outside of Savage currently, any job is easily capable of any content in this game right now despite what some would have you believe. Making one of the healers a test mule to see if a buffing rather than DPSing healer would be a good sacrifice imo.