Some mons, like Greninja, are used a lot, but their capabilities don't warrant that use. Others are used a lot, like the ones afforementioned, have capabilities that warrant that use. For instance, if we were to see teams full of mediocre mons all over the place, think Jolteon, Trevenant, Greninja, Charizard, Ninetails, Gastrodon, and other mons like these, the logical explanation for this is that people are "over"-rating them, and they think they're good when they're really, honestly not.It's weird answering some of these questions, cause you kinda have to guess the usage. I don't play often enough to see the trends of usage, other than fucking Rotom-W and Genesect being on EVERY GODDAMN TEAM, so a lot of the choices were basically "fittingly rated" by default.
As for people saying we have no clear/reliable usage statistics, use your experience playing the ladder. You obviously see more more of one mon than another. For instance, Rotom-w, Gliscor, Azumarill, Greninja, Gengar, Kangaskhan, Trevenant, etc. are everywhere in OU beta. Many of these highly used mons don't necessarily deserve this usage. As for OU bank, we see many Genesect, Rotom-w, Heatran, Ferrothorn, etc. and we face the same story. People are using mons that were good last gen, and mons that are new to see if they are up to par. Gastrodon is a good example of a previously good OU mon, whereas Greninja, Hawlucha, and Trevenant is a good example of people using mediocre, "sub par" new mons.