The Blues have three lines that rotate around like they are all first lines - each getting virtually equal playing time. He is playing well. He plays with talented players, including the rookie phenom Tarasenko. I think what had held back McDonald's draft position, and therefore his value, was that he lost the majority of last season to injury - he did score 22 points in 25 games, though.
Erik Cole, on the other hands, is not playing very well. I also suspect at some point The Canadians will remove him from the top line in exchange for Gionta, which will only further dampen his value.