Here's a case study for you: Carlos Gomez. 5 tool talent, and rushed by the Mets. He needed more time to develop, but the Twins also felt the need to justify the Santana trade, so he's the starting CF on opening day. Remember that experiment? Not only wasn't he ready, but the challenge meant he was taking his lumps at the major league level instead of learning things at lower levels where the team wasn't negatively impacted by that. Sano could turn into a star in this league, but it is highly doubtful that he's going to be hitting like one from day one. His first year is going to have some bumps. Rush him quicker, and he experiences more bumps at hte major league level and has his confidence drained when a half a dozen different coaches and players are all telling him how to improve his game. You aren't flushing a year or two of productive time down the drain because far more often than not, it is not productive time, and if you move the guy too fast, you cause other problems that arent' easily curable. The Twins have their fair share of problems, the speed at which they promote their guys is rarely one of them.
Here's a question... How many players have they had that have done nothing but dominate only to be held back? I cannot think of many. Slamma mabye... Neshek's and Bartlett's promotions took a bit longer than fans would have liked as both continued to rake in AAA while there were obvious needs at their positions in Minnesota.... But who else?