Potency and Tenacity - how do the mechanics actually work?

Umm, yes, okay, I'll try being more precise because that link doesn't answer my question about whether there is any point to 132% Tenacity.
Potency - Tenacity, yessss ... but if all characters who apply Debuffs are going to have their Potency cranked high by their players' mods, is there much point bothering with Tenacity?
If even a Tenacity of 132% doesn't make any difference, what's the point?