Tiers are based on uses, but only the main 3; OU, UU, and RU. NU is listed as pokemon that just don't have enough use to be used at all.
Now, within these tiers there are banlists; BL, BL2, and of course Uber. These lists do not take into account usage, but only what is deemed too powerful to reside in the tier below it. However, because there are so many pokemon in ubers, it has become a metagame all of its own. In the first couple generations, for example, there were only a few pokemon in ubers, which were not enough for an entire metagame. So that's why pokemon that see high usage, such as scizor, aren't sent to ubers; they're just not broken as deemed by the community.
Now, within these tiers there are banlists; BL, BL2, and of course Uber. These lists do not take into account usage, but only what is deemed too powerful to reside in the tier below it. However, because there are so many pokemon in ubers, it has become a metagame all of its own. In the first couple generations, for example, there were only a few pokemon in ubers, which were not enough for an entire metagame. So that's why pokemon that see high usage, such as scizor, aren't sent to ubers; they're just not broken as deemed by the community.