I have rarely seen that a problem in casual because the matchmaker seems to actually match what it considers support/tank against each other (healers and tanks, although some healers like SGE are definitely not tank material but well...), and also tries to spread melees and ranged around. This is however a mega problem in ranked at times because the player pool is too small so when it can't do that, it just does whatever, and it's a disaster. Try holding a point with BLM/SMN/BRD/MCH/SGE, that's pretty funny.