There isn't a "off healer" and "main healer". In the current optimal set up, there is a barrier healer, and a HoT healer, and this is the problem that Yoshi has specifically pointed out. Each of these skills along with ogcd heals when a healer's threshold is reached opens up the DPS windows for the healer to slam away at the opposition. The issue with balance is WHM has no distinctive advantages over AST or SCH. Though SCH regen capability is limited, its powerful shields and fairy negate this weakness. Plus, SCH brings raid utility. Like Grimoire pointed out, SCH doesn't really have any disadvantages at all. In fact, I find intense healing situations far easier to take care of with SCH than any other healer, including AST.

Tanks are actually much more balanced than a lot of people give them credit for, but PLD is a true off tank because of its toolkit. Both WAR and DRK are far better suited for MT when they are coupled with a PLD, and if DRK and WAR are paired with each other, they can go either way without a hitch. So the imbalance there is that PLD is forced into an OT role no matter what composition it's put in other than another PLD. If GUN is also designed to be an off tank, then this will give both of them an opportunity to be a MT when paired with each other. I wouldn't compare healers to tanks by any means. It's not even comparing apples to oranges. It's comparing apples to beef sirloin.

Balance with healers will come when they give all three distinctive advantages/disadvantages to each of them and leap frogging is eliminated. I don't think they can prune out shields and HoTs, but I think they can change how we use them. Giving all healers Barrier and HoT capability is a direction I think they can go, and then diversify from there. I believe this would also allow for a 4th healer when the time comes, but job design is not my forte. We're just going to have to see what they come up with.