As someone who use RDM and WHM, I can both relate and not relate.

Pro: My initial job is conjurer, so my job is to heal the whole team so no one died. And when I used RDM in a high level dungeon I still feel uncomfortable with tanks with HP below 50%
And RDM can raise the healer in SB dungeons and trials if the healer(s) happened to be dead.

Con: RDM is a DPS and WHM is a healer. You don't switch the two because the healer is too lazy to keep the whole team alive and lazily casts the tank Regen while expecting RDM to support themselves.