Seraphin was first picked just to practice with the team for a couple of weeks before leaving the team. They picked him to know what he could do and to allow him to know a little the NT with future years in mind. Ian is more known and knew more the team and that's why they didn't go with him for this practice spot. Seraphin was great during practices and the first game against Canada. With M. Pietrus and Bokolo out of the team, Collet has left the door open to Seraphin for making the 12 man roster.
The idea behind Traoré is that the team hasn't a low post scorer and could use his scoring ability at some point of the championship. But I agree that he sucks at defending and rebounding. His confidence shouldn't be a problem because he has had a good second half of season in Italy.