Well, the reason he keeps playing him isn't because Valencia is in good form, or indeed any form, are you disputing that? If not, what is your actual point?
Valencia has been played because there have been no alternatives, given they've all been injured. All we could have done was either play Young or Rafael at right-wing, which of course leaves left wing or right-back open - both of which we couldn't afford to fill given Nani, Smalling, Jones and Kagawa and even Anderson who can play wide left were all injured. We did actually try Young at right wing, forcing Giggs on the left, such was the extent of Valencia's poor form. Do you think Young would have been used on the right had Valencia been playing near his ability? Clearly we've played him because of a lack of other options, as well as trying to coax some form from him through a run of games, which hasn't materialized yet.
An in form Valencia is one of our best players, and definitely one of the best wingers in the league - Are you saying you've seen even a glimpse of that in his last however many games?