Well let me ask you this. In OU, for example, people use Politoed and Ninetales pretty commonly, obviously. But these two Pokemon are shit. They are only used because the benefits of their weather can set up menacing sweepers.
My question is, if Sand has equally frightening abusers (proportionally) in UU, then why don't people use Hippopotas more? My question is, if Sand in UU has the potential to be so good and centralizing, then Hippopotas being mediocre doesn't hold any water, because people are clearly willing to use bad Pokemon for their weather if the weather sweepers are good enough.
You could argue that Sand is good, but not that good. But that it would become that good with the entrance of Hippowdon. But then it's Hippowdon being broken, not the Sand itself.
edit: Whoa, I just realized how hard Zapdos fell in usage (in OU). Poor guy. :(