Tenacity is neither useless nor a garbage stat.
It is not the optimal choice of materia, but that is far different from being a terrible choice.

The extra damage mitigation you get from Tenacity isn't really large enough to make a noticable difference except in some very, very specific situations.
The extra damage dealt from having Tenacity is slightly less than that from having Crit/Det/DH, and since extra dps not only makes dps checks easier, but also makes fights shorter, the materia choices that give the best dps increase are generally considered best.
The difference is no more than a few percent though, so it really doesn't matter much which materia you use - you will probably not notice the difference anyway.