Maybe an exception can be made for Tentacruel.
It may be that the original purpose of UU was to create a metagame utterly distinct from OU so that lesser-used pokemon could see play. Eventually each tier reaches a point where it revolves around an elite tier of key pokemon that dominate it, and at this point, bar the few pokemon that hover on the tier boundaries, the players of each tier become familiar with the uses of the pokemon commonly seen and available to them.
Then, innovators in OU notice that Tentacruel has many uses in the OU metagame, and others pick up on it so that it becomes widespread. When this happens, by the strict definition of the metagames Tentacruel is no longer legal in UU.
However, the only reason why usage was so important before was because we didn't know how powerful the pokemon were relative to each other. After the metagames are defined, when suddenly a pokemon well within UU sees significant usage in a higher tier, we still know that it is balanced enough for the tier and we may even remember that it was once a pokemon that the tier was associated with.
Perhaps Tentacruel only sees so much use because of all the Obi stall team doppelgangers, and if we only looked at the people that really used Tentacruel for its talents it would remain UU.
However, that is irrelevent to my argument that the usage argument should really only apply when a metagame is being defined or potential metagame breakers are being tested. After the major testing is done, we already know everything in a tier is balanced so why suddenly ban them based on a change in usage?
To allow Tentacruel in UU would hardly change the metagame into an image of OU. Furthermore, one could argue that Tentacruel will always fundamentally be UU and not even a short sojourn in the spotlight can change that.
