If you add McKay who needs to be close to the rim, then you add McKay's defender close to the rim as well. Suddenly spacing the floor to open easy drives to an unguarded rim is nullified. If you take McKay and send him out to the perimeter to pull his man out there then you have a guy that basically is guaranteed not to be guarded which leaves his defender free to basically play center field and wit to block shots. Suddenly, the "five guys that can score from anywhere on the floor" goes out the window. McKay will be a big help on defense but he will bog down the offense at least some.