Bad wording on my part sorry.
I don't mean that the different healers are required for specific content, but rather each healer brings a niche with different content taking care to reward said benefits of the different jobs within a role.
Midas did a pretty good job of this. Plenty of mechanics forced healers to split from their group, often for a pretty significant chunk of time. This gave Medica II really quite significant worth as you could generally still reach the group with it. Meanwhile AST's 3.2 cards weren't as broken as 3.4, but it still offered a noticeable DPS benefit, however this had to be weighed up with the likelihood of requiring more healing from your co-healer to compensate for AST not being quite as good at dealing with jail time etc.
Switching between the two jobs as I progged through the tier was super enjoyable and something I miss doing in recent times.
One thing I think you are overplaying though, what is there to reskill as a healer? Min maxing melds? Or do you mean reacclimatising to each healer nuances? If it's the later, then that's entirely the point. It really does keep things fresh.