Actually, I'd argue otherwise. WHM is far more useful than a RDM in curing and buffing. With the addition of Shellra V, you get an additional 3 MDB and up to 7 full merited which is over a 5% loss in magic damage. That might not seem like a lot, but it adds up over time compared to other jobs. To add to that, WHM gets +20 to bar spells from legs and another +10 from full merits which can significantly help reduce certain elemental damage like from JoL or Mijin Gakure. So in effect, we add to the defense of the party more so than other jobs. That's not all. WHM gets a large amount of -enm which will help hate in long term fights whereas RDM could get a decent amount, but usually do not use. We can also sub BLM for ES if required without loss of much needed spells, -na, erase, etc.