I think who main a DPS will not likely run a tank or healer, and if any one-time reward for it, they will just to reap the reward and return to dps thereafter.

And adding healer and tank classes does nothing to queue too, imho, maybe at start more people playing them, just to look at the new shiny, but in the long run the queue returns as it were left off before adding those jobs, because who likes dps role will play it, same for tanks or healers.

If really the tank shortage wants be filled, it needs new players in game that want main the tank. Existing players will main the role they chosen, hardly I can see switches.