There are other off the court considerations as well. Irving is likely to have a longer term impact on Duke's team/recruiting then say Scheyer because he is A) likely to be an nba all-star and B) is such a dynamic personality. With Irving coming back each summer to work on his degree, he will be around the current players and at certain points potential recruits. His presence can not only impact recruits but since he will also be working out, can lead to improvement of the current players. Similarly Smith (even though less successful in the nba) is the kind of player that will be helping Duke out for years to come even though he has graduated. Some players are like magnets and will help attract other high quality players. I don't know if Parker is that kind of personality but if his game is as good as advertised, it may also lead to others following his footsteps.