>>31841902This is straight from the horses mouth.
http://www.smogon.com/xyhub/tiers"UnderUsed, or UU, is the tier that contains all of the Pokémon that are just short of OU in terms of the required usage amount, which equates to a 50% chance of encountering the Pokémon in OU in 20 battles. These Pokémon can still be used in OU, and a lot of them are actually quite powerful as well, but they simply aren't used enough for them to change their tier placement."
UU mons can be used to great effect in OU, they just aren't used enough in OU to be classified as OU. Which is the position Nidoking is in now.
