It's not about what he's won in a team of fantastic players, it's about what he can offer as an individual. Walker is vital to us in terms of pace, power, aggression and pure determination - plus he seems comfortable enough where he is that he's not been bitching about being dropped recently because he knows he'll get his chance again when needed (Champions League).
I'd go as far as to argue that Walker is one of the most irreplaceable players in the squad in terms of what he offers being so unique.