Yea this isn't exactly right. If a DRG can do 400 single target DPS on one mob that is fine. If that same DRG can only do 150 DPS on 1 mob using AOE attacks, then thats bad, BUT it becomes on-par (not accounting for TP) at just 3 mobs. Now when the Tank has like 6+ Mobs, all of a sudden that DRG doing 150 DPS a mob x6 is now dealing 900 DPS, which is now justified with the higher TP cost. Personally back in those Brayflox HM runs of old, I used to do over 1400 DPS (no joke) to some of those bigger trash packs as DRG.
Now granted MNK aoe is very very bad no matter how many mobs there are, but from my understanding NIN aoe is fairly decent, as is BRD when were talking 5+ mobs, and although the "sweet spot" for SMN is 4 targets, MiasmaII, Shadow Flare, and Tri-disaster has no cap like Bane does, so there dps increases as mob count does as well. Now granted if the over-all aoe dps of the group is low then your DPSers will run out of resources (TP/MP) to do dps and then THATS when you know your tank has pulled more than the group can handle.
so tldr; if your group has resources available, its faster to AOE than it is to Single Target. If you are going to be using AOE dps, then the more mobs you pull, the less resources and time you will use overall.
Also want to comment to his:
Those 3 points you made are actually kind of contradictory. You say its stressful and risky, and yet also boring. If your stressed out then your really not bored.. your stressed. I like playing both DPS and Healer when it comes to big pulls. DPS is pressure to kill things fast before the Healer/Tank run out of cooldowns and die. That pressure for me is the challenge, and with challenge comes fun. What fun would I have if I just spammed 1-2-3 with no threat of anything bad going to happen? When I am healing, there is the pressure to keep the tank alive. If that is easily managed, I can force it to be harder by adding my own DPS to the mobs. And again, challenge brings fun, and I like that fine line between healing and dpsing. Now as tank... I don't really enjoy as much as the others truthfully because of people who like to complain about-every-single-thing. I got you into your dungeon, now get off my back unless I am being absolutely terrible. But even when I do tank I enjoy challenging myself. I could pull 1 mob... and again 1-2-3 until its dead but where is the challenge in that? Pull more and juggle threat management and damage reduction.