Why don't people like to pick/use Tenacity over Determination?

[Warskull]

I was wondering if I should do a tenacity meld for Ruby EX. Seems like it might be helpful for the ad tank buster and for the meteor.

Tenacity shines with the adds' tankbuster in phase 2, particularly during progression. That tankbuster hits extremely hard and a missed cooldown or a healer having you not quite topped off can easily result in your death. Some tenacity really helps smooth out mistakes here. There is a ton of incoming damage and mistakes by the DPS will have your healers struggling to keep up. Eventually you get to the point where you don't need it anymore, but it helps on the road to learning the fight.

With the meteor it doesn't help so much and your healers clean that up with AoE. Proper stacking is necessary here.