I don't doubt Seattle preferred Wade Miley to Clay Buchholz (IF given the choice) but I would note that Miley and Buchholz share the same ERA+ of 77 this season.
Advanced metrics give the edge to Miley:
WM 15 G, 15 GS, 87.1 IP, 5.36 ERA, 1.40 WHIP, 0.0 bWAR, 0.2 fWAR
CB 18 G, 13 GS, 80.2 IP, 5.91 ERA, 1.49 WHIP, -0.9 bWAR, -0.5 fWAR
Neither has provided desired results.