Balancing tanks and healers is currently tricky because they inadvertently designed the first two jobs in these roles is such a way that adding additional jobs in a balanced manner is not really possible unless they rebalance things.
They were designed such that A+B > A+A or B+B with A>B in various aspects. This is fine when there is only 2 jobs in a role, but troublesome when C is added. You want A+B=A+C=B+C>A+A, B+B or C+C, but we ended up with A+C>A+B>B+C>A+A, B+B or C+C. They need to get it so that A+B=A+C=B+C before they add D.