and the "strictly DPS" notion is mostly wrong. a RDM is regularly supposed to use most lower tier White and Black Magic. DPS can't regularly use Cure spells in this game, that hurts DPS. the sword part of RDM makes them less suited to be a back line healer, so you're left with "are there enough tricks in their bag so that they can be viable tanks". between anything you can do with "Spikes" spells, Phalanx, Parrying with a Fencing Sword, Fast Cast, Stoneskin and Cure. i don't see why not.
