Potency and Tenacity - how do the mechanics actually work?

Yes tenacity serves a purpose. You would need to collect data to present a real picture of what is going on, and pay special attention to debuffs that cannot be resisted.

The down side, as was being pointed out above, is that you need a really high tenacity, just to start getting away from the base resist amount. So it's a fair amount of work for little payoff, as you are only reducing the chance of debuff not really making headway on a full resist, by any means.