If groups start preferring one combination more than another, SE will make adjustments. Remember when people didn't want warriors in coil? SE eventually found ways to balance the two...

So, that will happen with the healers with the addition of a new healer... and the tanks with the addition of a new tank...

Just as the dps were adjusted with the addition of a new dps.

Right now, WHM + SCH are the perfect team. Scholars are preemptive, and mitigation-minded. White mages are reactive (although it does help to know the rotation) and huge heals heavy, although they don't have much mitigation (besides stoneskin, which is a longer cast than adloq and not a heal). The addition of astrologian will mean...? We don't know what direction they're taking this in. Who would have thought SMN would be a dot class? As others have said, all classes/jobs will be obtaining new abilities, and those abilities will probably help us understand the balance they plan to implement.