Quoting: RazWild
Granny, Haula and Kuemper definitely all needed more time down in the AHL to start out with. Brodin is the only one there who was pretty much ready to go right away.
Only Dumba had more than enough AHL games under his belt, but probably could have benefited from an additional 20-30 games down there.
Kemps problems were in his head. Being in Iowa longer wouldn't have fixed that. he was .930+ sv% in the AHL, came in and was .915 sv% in the NHL. Harding's MS is what got him called up and he ran with it. He also preformed well in the Playoffs that year, getting injured sunk the team vs CHI.
Granny had nothing more to gain from the AHL. He was a PPG, had already been playing is a men's pro hockey for a few years before that. Trying to make him a center, when he was a natural winger was the mistake MN made with him.
Haula was an injury callup, and forced his way into the lineup. He made the team better and looked like he belonged in the NHL (he lead all FWDs in +/- that season). The bottom-6 was a disaster without him in the lineup.
Prospects don't develop at the same rate. Some need no AHL time, some need half a season's worth, and others need multiple years. It's the (non-elite) kids from the CHL that tend to need longer in the AHL compared to the guys coming from Euro pro leagues or college. The competition level is less of a jump from them into the AHL.