You'd have to really hate DIPS theory to jettison Miley, career FIP 3.80 and with two years remaining on a 3/$18 deal, in favor of Leake, career FIP 4.21 on a more expensive deal.
The appeal of Leake is that he's really young (28 in 2016), and like Miley, he takes the ball reliably and throws innings. But he strikes no one out — like, meaningfully fewer than Miley, who's hardly a strikeout artist.
Basically, Miley has value because he's better than Leake, and only a year older, and Cherington signed him to a highly team friendly deal. Teams that are wrinkling their noses at giving Mike Leake 4/$60m should want to offer meaningful value for Miley at 3/$18, with an option for a fourth year at $12m.
There's a scenario, now that the wallets seem open on Yawkey Way, in which it makes sense to sign Leake, deal Miley to e.g. the Cardinals or the Giants or another contender on a budget who needs starting pitching, and basically use that as an avenue to turn money into prospects, either for development, or to flip for Strasburg at the deadline when the Nationals disappoint again (or whatever). But in a world where we're running out of 40-man spots and high-minors playing time, I'd probably only be interested in individual high-impact kinds of prospects, and I'm not sure I see a return that would make sense.