I think we need to look at the bigger picture here. Starting at level 21, WHM approximately receives a new cure spell every 20 levels:
Cure III - level 21
Cure IV - level 41
Cure V - level 61
Cure VI - level 80
I take the jump of only 19 levels from V to VI as a hint that the devs intend to give WHM Cure VII at level 99. At that point, it should not represent a balance issue at all to give RDM (and SCH) Cure V.