I'm kinda the opposite here. For a long time I used tank solely for DF for fast queues and bonus gil, even though I prefer healing. Only reason I hung up my axe is now I'm working on Thyrus Nexus so every dungeon counts. I didn't just phone it in though, I did my best to play to the best of my ability, to do any less would be inconsiderate.

That said, my time tanking and healing has shown me one thing: I can't trust DPS. The number of times they've ignored important adds (like the bird,on Midgardsormr, everything on five-headed dragon...) Killed themselves by attacking when they shouldn't have (oh hey, five-headed dragon again...) Directed boss AoE to the wrong area (first boss of AK HM and KotL)... just any mechanic you can name, they fail again and again. Seems they've taken the age old advice of "you're invisible as DPS, nobody will notice if you mess up" to heart.