I don't know if I fully believe this, but I can see some merit. A few years ago, Ryan Howard was one of the hottest minor league prospects. The Phillies kept him in the minors an extra year and a half - he was 26 in his first full major league season. I think their philosophy with him (other than having Jim Thome blocking him) was that they would have more years of Howard's prime by keeping him in the minors.
Fast forward to our situation. With the number of highly-regarded minor-league prospects we have, keeping them down 1/2 a season longer will control costs and allow the Reds to control them during more of their 'peak' years. With the arbitration salaries spinning out of control (see Howard = $10M!!!), it becomes even more important to have guys for as much of their peak years as possible.
Howard was the one I thought of with a team keeping a top minor leaguer down - are there any other examples anyone can think of? Is it worth keeping a guy down a half or full season to be able to have him for more of his prime?