Quite honestly, a single PLD can tank + stun that fight without difficulty.
The Eruptions are on extremely predictable patterns, so there should be no reason to just sit around waiting to stun, and thus you are hardly simply a "stun bot".
If you have an off-tank stunning, they should be DPSing their face off (as best a tank can, anyway) between Eruptions. If you get too focused on DPS before Hellfire, it's not even a big deal because Eruption is pants until the last half of the fight.
I actually feel the opposite the Stun mechanic. Other games have every class toting off-GCD interrupts on short timers, which trivializes all casting mechanics on any enemy which isn't immune. Thus, all bosses wind up immune and interrupting ceases to be a mechanic for the bulk of endgame.
On top of that, I like the trade-off between zero-CD on-GCD stun and high-CD off-GCD stun. Certainly a lot of groups will tend towards a PLD "stun bot" but quite honestly most competent FCs running endgame will look for more risky strategies that permit them to bring more DPS or healing to bear on any fight that doesn't explicitly require 2 tanks.