Actually, it's party due to game design on party composition. Example - Titan HM. Does it really need 2 tanks? Nope. If you understood the fight (and of course, decently geared) then it's obvious that there is no tank swap or other mechanics that forces you to use 2 tanks. But if you DF it, you will need 2 tanks. Why? IDK, that's probably for Yoshida to explain. Maybe just in case the MT is a flop and dies, the OT can pick up the boss. It allows you to have 2 ok-ok tanks and swap each other in case one dies, but in actual fact, if you have one good tank it's good enough. In this case, there isn't really a stress on the need to have a good tank.
But if it's a 4 man dungeon like Pharos Sirus, then yes your tank needs to be at least decent. Why? Because it's a 4 man, Tank-DPS-DPS-Healer composition. If your tank dies, there is no tank at all, and of course a wipe will occur.
Again, let's look at party compositions again, and back to Titan HM as example. We talked about how it actually can be done with 1 good tank. It can also be done with 1 good WHM, provided that DPS are not eating damage other than tulmults and your tanks aren't dying. But DF forces you to have 2 healers, just in case your DPS/tanks are a flop and dying. Then we look at DPS. 4 of them? Can it be done with 3 DPS? It can be done. I'm not talking about wearing full i90 sets and carrying - back then in 2.0 there are already people selling HM runs in DL gear. There is a video out there showing Titan HM done with 1 tank, 2 DPS and 1 healer (albeit I believe they are overgearred). What does this say? You don't really need 4 DPS for this battle either. If you have 3 good DPS that can squeeze out every DPS he/she can throw out, it's good enough. But the game says 4 DPS just in case some DPS dies or the 4 DPS are just ok-ok only.
And for the fact that the system is structured in a way that only Tank classes are able to absorb boss's onslaughts, only Healers can heal as required, only DPS classes can deal decent damage, there is no other choice. Tanks responsibilities is to ensure that they know their job and use CDs correctly. DPS's job is to make sure that they don't eat unnecessary damage and be a burden to the team while killing the mobs/boss ASAP (because the longer you drag, the more room for mistakes for everyone). Healer's job is to make sure everyone is topped up. It doesn't mean anyone is above another in responsibility and importance.
If once day there are changes to this system, such as introducing hybrid classes, then people may think differently. Example, Blue Mages that can cast offensive and healing magics at 80% efficiency of their specialized counterparts, Black/White Mages can do. Maybe a Beastmaster that have abilities of a Warrior class at a lower efficiency/potency but at the same time, have the potency say 80% of a melee DPS class.
When this happen, even if in a 4 man dungeon, will I care a deal if a PLD/WAR dies? Maybe not as much, the Beastmaster can take over until the healer raise the PLD/WAR. Is it all lost if the sole healer dies? Maybe not, since I have a Blue Mage. But we don't have mixed classes now, so it is important that everyone understands and does their job well to make everyone's life easier.