It's summer league. Guys can put up huge statlines and fall flat on their face in the regular season and have a bust career.
The best part about summer league, is being able to watch all NBA teams play and see if anyone shows enough potential to sign and pick up for the practice squad(as many SL players are available for pickup). Then you go from there, if your practice squad guys pan out, you give them a roster spot/7-days/D-League with hopes they might keep improving.
As far as Portis, he'll get time to show his stuff on his rookie contract. The FO gave up a lot for McDermott, so he's going to get a lot of benefit of the doubt regardless of SL play.