It seems to me AAA should be the final proving ground for prospects. But looks like most rosters made up of career minor leaguers and former MLB players. There are a few high prospects but not many. Is it to be prepared in case of a series of injuries at the same position, so not reliant on rookies. Don't want to start high prospects service time early if injuries occur? Seth's article "Looking ahead" lists 21 players that will be rule 5 eligible at the end of the year as either "locks, possibilities, or bubble" to be added to 40 man for Twins at end of year, I would think you would want to see them at the AAA level at some point this year to help make decisions at the end of the year. Also, use AAA to evaluate 14 players on 40 man that don't make 26 roster.

Most of the top flight prospects are at AA.  AAA is for players who are regarded as immediate help for injury or lack of performance at the major league level.  Also players who have already dominated AA and have nothing more to learn at that level.