reb, my view is that pitchers are different from hitters. A pitcher is the initiator, and you can scout his stuff/command. His velocity/curveball/command don't really depend on the level. The plate is the same size and distance in AAA as in AA.
*Most likely Johnson's control and consistency are NOT nearly good enough for the majors. But *if* they think they are, I don't expect that some starts in AAA would change any of that. Either he's ready, or he's not.
While a pitchers ability to pitch won't change from AA to AAA, a hitter's ability to hit often does change significantly with each step, because the pitching gets better with each step. And since scouts haven't seen a guy at AA need to face AAA-caliber breaking stuff, much less big-league breaking stuff, it's harder to evaluate whether the guy's hitting ability is ready or not. And, a hitter can't practice against better pitching until he faces it. A pitcher can practice his slider on the slider as often as he wants.
Granted, the "face" issue is more complex; and requires personality evaluation. Maybe a guy had velocity/movement/curveball/command/consistency in AA, but will fall to pieces when the adrenalin rush of the big stadium hits. Or, maybe he'll throw a couple of the same pitches he threw in AA but they'll go for HR's in the majors, and then he'll fall apart and all of the command/consistency that looked fine in AA will go to pieces.
*I suspect that's very much a risk for Johnson, and even more so for Edwards. When a guy has shaky control to start with, amping up all the pressures in the majors likely exacerbates that.
But certainly there's no question but that Edwards stuff. His velocity and movement are more than plenty good enough to excel in the majors. It's the control that's obviously limiting and worrisome.