Anybody who wants to play some form of GSC UU, to be honest. PO actually has a UU ladder, which gets maybe 3 battles a month, but it's still something that comes forth and says "I dub this team UU". There were also a couple minor UU tournaments a few years back that used the PO UU tier as the banlist, and matches from the earlier of these tournaments led to PO finally banning Kingdra. As meager as this is, there's really nothing equivalent setting precedent for the Smogon-specific version of the UU tier.