I'd love it if Miller would develop a cutter, as long as he doesn't become overly dependent. He works well off his fastball right now. I don't know if he'll ever be a Lester, but he's much, much, much better than DiceK. DiceK hitting the DL and Miller replacing him is the best thing that could have happened to this rotation. If we slot Miller in the 4 slot, and have a core 4 of Beckett - Lester - Buchholz - Miller, that's a huge rotation. Lackey, as a 5th man, won't ever live up to his contract, but if he can settle down and throw to a 3.90-4.10 ERA the rest of the season (not overall ERA, just ERA from today to the end), then this rotation just became stellar.
As far as keeping Miller on the roster, you gotta think that he is craving some stability. The Sox took a chance on him, if they extend him for, say, 3 years, it will probably be 3 years/12mm or 15mm. Something really cheap considering all he's been through, and he will take that in an instant. Comfort, a slot in the rotation, amd $5mm is more than he could have dreamed of just 2-3 months ago.