Can we just admit already that tenacity does basically nothing right now?
Now hear me out. I'm not saying that it doesn't reduce the duration of most CC. That's not in question in the slightest. Of course it does. What I mean is that the functionality is currently lost because of the state of the game.
See, tenacity is a fighter stat. You can argue that tanks can benefit from it too, but tanks usually have something that makes them not care if they're CC'd. Maybe they have constant AoE damage regardless of their ability to perform actions, or are looking to eat your CC in the first place, what do they care? If you're expending valuable CC on them, you're not using it on the people that will actually suffer the ill effects. Any squishy that has the stat is unlikely to survive the duration of the CC whether it's 2 seconds or 1.3 seconds regardless. So my argument is it's a bulky fighter stat...what do they call them now...uh...juggernauts I guess? I think juggernauts, but it applies to any fighter that's going tanky.
Characters that are disproportionately affected by CC because they want to be in the middle of everything doing damage, but lack the tools a true tank has to survive long engagements or big CC to tip the scales in the favor of their allies.
But right now, the thing is, be you anything but a true tank or glass cannon currently, you explode within the first few seconds you enter combat, and without some ridiculously strong defensive tool, you may never see the back line before you melt. Tenacity is supposed to be most effective in longer, drawn out engagements when you're only facing 1 or 2 CC that might come your way. But chained hard CC, something I've been seeing more and more of, is still going to have a similar/identical effect on your character, it has no realistic counterplay. You're not going to be able to do anything for 3-5 seconds OR MORE, no matter if you have 50 tenacity, or 0. And it's certainly not going to do anything against the ever-present knock ups that are becoming more and more numerous over time.
I had an idea for tenacity some years ago now. Where its effect was more to counteract chain CC than any individual effect. Something where you have to be careful to not hit the fighter with too many weak CCs.
I wanted it to be "if hit by a CC effect while in combat, reduces the next incoming CC by 35% duration, stacking multiplicatively" So first effect is 100% -> 65% -> 42% -> 27.4% ... etc. Kinda what they do in MMOs to prevent chain CC in other words, albeit somewhat less effective than that.
Since we all know Riot isn't likely to lower damage numbers anytime in the near future, this seems like a decent compromise to -maybe- push
closer to the usefulness of 
etc , and they go
,my skills have very low effect on them to do comboes, and i usually lose.