Potency and Tenacity - how do the mechanics actually work?

That's kind of my point.
People are modding their characters to be able to hit the character's who have such insanely high Tenacity. So for those who stand no chance of getting there, is there any point even trying?
It's reached such silly levels that for most, I don't see it as being worth it anymore. There's no point. My Juhani's Tenacity of 132% comes from 6 Tenacity Mods and one of them gives +24% Tenacity. If with all that, she gets a relatively low Tenacity by today's standards, what's the point?