I have no doubt that exit velocity could be useful for analysis purposes. But at the same time it could be quite useless to the average fan. Personally I don't think it will ever add anything to my baseball experience.
It's just part of the technobabble of modern sports. In golf now you have 'clubhead speed', 'launch angle', 'spin rate', and 'ball speed'. It's not good enough anymore to just say the guy hits it a f'king mile.
I think there's another team or two out there that would take him if they didn't have to give up much in return. As Sox fans we've been watching the guy so long that all we see now is his downside.
For what it's worth, over the three seasons from 2013 to 2015 Miley posted 5.9 fWAR and 4.6 bWAR while Buchholz posted 7.5 fWAR and 5.4 bWAR.
I agree with your other points. Miley had significant advantages other than the quality of his pitching.