The world first race folks melded tenacity for insurance. Tenacity is blown out of the water by a couple weeks' worth of Def/mDef increases through simple ilvl upgrades.

The idea of a defensive BiS is pretty ludicrous actually, because by the time you reach it, you're so overgeared that no defensive attribute is netting you any real gain.

BiS is synonymous with damage, because the value of DPS scales with progression, from learning, proging, clearing, farming, speed-running, and beyond.