You would prefer to give players at least a full year of AAA ball, very similar to AHL, AA and below is more like juniors.
In AAA ball, you have a lot of veteran AAAA type pitchers, who lack ML velocity but have secondary pitches and command. If a hitter gets to AAA, usually he can handle a ML FB, it's learning to recognize and hit (or not swing) at curves, sliders and changeups that's the key to ML success.
Pitchers need that year of AAA ball to refine their secondary stuff, pitching prospects for the most part have ML FBs, but lack command or secondary pitches they can throw for strikes (and the consistency not to hand curves or sliders over the middle of the plate belt high). It's a lot easier to throw changeups to hitters who'll miss them when you don't locate or signal your pitch through a change in arm speed. In the Show, those mistakes tend to travel "federal express."
So it's a good thing the Phillies aren't rushing guys, you can see watching Thompson and Pivetta that they're not major league ready, and Hoskins only has 100 or so AAA PA - look at Joseph's struggles because he missed 1000 PA due to those concussions.