Not entirely, but that is part of it. It's just as much the fact that tank/heals is more "work" and more responsibility. Also more accountability when things go wrong. I play DPS and tank; I love my PLD and I love tanking, but when I'm tired or burnt out, I will run dungeons on DRG because its honestly less effort.
I don't agree that tanking/healing is more work in this game. If DD's are taking damage because they're not constantly moving out of AoE crossfire or not delivering enough damage soon enough, wipes happen. I'll cite an early example so everyone will understand. Take the Ifrit fight where the DD need to kill the pillar thingy before he uses Hellfire. If Hellfire is going off, it's the DD's fault. There are many other examples to list, but let's not dwell on it. As for healers, their primary focus is the tank. Tanks need to hold hate and stay alive. Marking IMO doesn't have to be a tanking thing. Anyone can mark so I'm not adding that to a tank's role. DD's often find themselves dying if they take any damage because the healer is so focused on the tank. This mean many of the deaths are a direct result of the DD being stupid.
No offense, but I wouldn't want your DRG in my party. If you think it's easier to play a DD, you're doing it wrong.