My stance on Tenacity is that Tenacity is a bruiser's stat, as opposed to a tank or glass carry's stat - which is why you don't really need many options for it. (I define a tank as primarily CC-oriented, so as far as I'm concerned Champions like Shyvana and Mundo are bruisers - they offer sustained damage, not CC).
As a tank, your contribution to a teamfight tends to come in bursts of utility based on your cooldowns, on top of simply being tanky. Your job is to blow cooldowns to either initiate or peel, so you don't have much sustained contribution - you don't do much without your cooldowns anyways, so you don't lose much by being CC'd. Generally I'd even say that, as a tank, you shouldn't expect much CC - if they CC you instead of your carries or bruisers, you've already done your job. Because tanks have burst contribution and are too tanky to be focused, Tenacity (or any CC reduction) probably won't help much - this is why I agree with Riot's decision to remove the tenacity from Golem.
Bruisers tend to contribute sustained damage and tankiness. That means that if they get CC'd, they'll always lose out on their damage contribution, so it's valuable to reduce the time of CC on them. They prefer Tenacity over cleansing actives because their defensive itemization means that they'll get hit by, and survive, multiple waves of CC.
AP carries (Mages and AP Assassins) tend to contribute bursts of damage based on cooldowns, but they're also fragile: you're a liability when your skills are on cooldown. Generally, if you get CC'd as an AP carry, you're dead even with Tenacity - the difference between a 2-second stun and a 1.33-second stun is minimal when you're bursted out in 1 second. Because they contribute based on cooldown rotations and are so fragile, AP Carries will choose the stasis of Zhonya's over Tenacity any day. You're useless without cooldowns anyways - might as well spend that time being under an untargetable self-CC rather than getting caught by an actual CC.
AD carries have sustained contribution, but are also fragile. Fragility means that Tenacity won't help them much, but AD carries' sustained damage means that they desire CC reduction. This means that they prefer cleansing actives: it reduces the duration of a single CC to practically zero if used perfectly, and they get to keep their constant damage. Mercurial Scimitar and Mikael's tend to do this job well, but the reaction-based actives mean that you don't see them much outside of competitive.
AD assassins seem to have a wonky spot: they're fragile and contribute in burst (like AP carries), but don't have Zhonya's. Same story for sustained AP carries like Kayle and Cass - they're fragile and sustained, but don't have cleanses. I'm going to presume that these champions; individual kits have ways to counter this deficit, though. In any case, their fragility means that Tenacity isn't the answer.
TL;DR CC reduction is preferred mainly by Sustained champions who have the Tankiness to survive multiple waves of CC - which is why only Bruisers really need it. Sustained-Fragile champions are usually AD carries who prefer the one-time save of cleanses; Burst-Fragile champions are usually mages or AP assassins, who don't have anything to do while their skills are on cooldown (and so prefer the stasis of Zhonya's); and Burst-Tanky champions are usually tanks who have nothing to lose to CC anyways. Exceptions tend this rule tend to be rare, and rely more on their kits to work than itemization.
If Riot introduces any new Tenacity itemization, I believe it should be directed exclusively towards bruisers over any other class.
Exactly what Tenacity items should look like in terms of stats, cost, and trade-offs is an entirely different question.
available in SR would already help alot, and maybe adding tenacity to warmongs so tanks/bruisers have another tanacity option besides mercs while Warmongs gets the little buff it needs
doesn't have Tenacity anymore...
