The defense strategy with the psychologist may be two-fold...to explain the "creepy letters" AND as a precursor to Sandusky's testimony. If Sandusky does testify and if he is rattled or demonstrates any of the tendencies testified to by the psychologist or tendencies shown in the Costas interview, then the defense will use the disorder to try to explain away that behavior. Just my analysis of a possible argument to be made by the defense.