I don't think it's as much that a AAA 325 hitter isn't ready enough, it's whether after 25 AB he's really a 320 AAA hitter. It's all about the sample size right now - exactly what we saw with JHM, who after exploding out of the gate with a month at 1000 OPS then promptly put up a 2nd month at 700 OPS.
The 'working on his defense' argument I'm less persuaded by. The validity of that argument lies in not wanting your developing player to be costing you major league games as he learns his trade, but that assumes you have a competent defender that would be playing there if he weren't. For much of this season that hasn't be true - maybe it will become more true if McKinstry or Ibanez establish that they can handle 3B decently if one of them starts playing it more. Schoop may still have the defensive chops but a bat like Schoop's is exactly what drives the need to call up a Keith.
It is true that with the new MiLB scheduling system, MiLB players travel less so theoretically they have a more time to practice. I guess there would be something to be said for that.