The "tenacity" Problem (other melds aswell)

[Absurdity]

In my mind, stats like tenacity, and piety should make you better at your role, and not be damage related -- there are already three stats that do that. I often wonder what it would be like if stats like tenacity and piety reduced the cooldowns of defensive, and healing abilities or something along those lines.

They would either be just as useless as they currently are or mandatory.

Since fights are entirely scripted you know exactly when tank busters are going to happen, so you'll either get scenario
A: Tank busters happen roughly every 60-90 seconds, meaning you will always have enough defensive cooldowns available by default, that's the scenario we currently have, making the "new" Tenacity completely pointless.
or
B: Tank busters happen so often that will need to invest into Tenacity to reduce your cooldowns to the point where you are able to mitigate each buster, making the stat mandatory or you'll simply kiss the floor.

The problem with non-damage substats is that they share the same pool as damage substats, if you invest into Tenacity it consequently means you're losing that amount from a damage-related substat.
So if fights then do not require the defensive substat it means your investment is useless, because the other tank who invested into a damage stat can perform the job of "not dying" just as well but also contributes more to killing the boss.