I play SMN/BLM/PLD/WHM depending on the situation and what I'm in the mood to play.

-WHM if there's only 1 healer in the party; won't switch if there are no other healers.
-BLM only if I know my opponents aren't good. Good or premade teams = a useless BLM no matter what anyone says.
-PLD if there are no tanks in my party.
-SMN whenever I feel like my team's dps is trash.