If you're going by usage stats, Yi is always up there because sub-30 players love him. Yi more or less disappears completely in all games modes above a certain number of games played.
Where are you getting your stats? Because they're probably bull**** to begin with.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.