Forget Porter's scoring ability for a minute. Imagine that it takes two years to get up to speed (I don't think it will, but for the sake of argument,let's just say his .550 2p% and .422 3p% don't translate immediately). The main reason Porter will provide valuable minutes off the bench from the minute he steps off the court is the almost completely unrecognized fact that he was arguably, inch per inch, the best rebounder in college basketball last year.
At 6'8, 205, the guy is not exactly Charles Barkley, and he played small forward in the Big East, which as always was packed with wide-shouldered action figures crowding up the lane. And yet, Porter, when asked to do so, appears to have been able to transform into an elite rebounder at will, often dominating larger, more widely acclaimed specialists. His average, 7.1 rpg, was good enough for 9th in the Big East, which is remarkable on its own, but the numbers are a little deceiving in that they do not show the number of times he went over 10 rebounds (7 times). On days when the shots weren't dropping, Porter was everywhere.
Look at what he did to Louisville on January 26 of last year:
Porter's 12 rebounds jump out of the box score, especially against a team that dominated opponents in the category all year. In a game in which Porter did not shoot well (17 pts on 5 of 13 shooting), and was uncharacteristically sloppy with the ball (7 turnovers) he was sublime on the boards. And this against the hulking 6'11 Gorgui Deing, who finished 2nd in the big east in the category, and was drafted 21st by Utah on rebounding and defense skill alone.
With a team that likes to shoot the three (Wiz is 8th in 3pt attempts) Porter will bring immediate value in long rebounds away from the basket, and will surprise a lot of bigger, more athletic guys.