At worst (well, the "worst" that assumes he is fully recovered from injury) he provides some usable depth. A ~12 PER is poor for a starter but isn't too bad for a reserve. If he can be a solid backup, he'll upgrade the team's depth and allow them to use Outlaw more at power forward (which is where he's generally played best).
You beat me to it. I was going to make the exact same point. Martell will be 10th or 11th on our depth chart. How many teams have a 22-year old with experience as a starter in the NBA as their 10th or 11th man?
When considering PER, you must also consider the role that player played. It's much harder to maintain a high PER as a starter playing 28 - 30MPG than it is as a back-up playing 12 - 15MPG. As a 20/21-year old starter, Martell had to play the bulk of his minutes against some of the best players in the league. Defending, and being defended by guys like LeBron James, Carmelo Anthony, Paul Pierce, Ron Artest, etc. is a lot harder than racking up stats against other teams bench players. For this reason, assuming Martell is coming off the bench and playing many of his minutes against other bench players, I see him posting a career best PER this year. I would not be surprised to see him have a PER > 15.0 this season, but for a 10th man, I'd be happy with a 14.0.
Also, I think the players he's on the court with will also have a positive impact on his PER. Martell has never had the benefit of playing with a center that commands double teams, or a point guard that is good at creating for others. The centers he's played with, as a starter, have been Theo Ratliff, Joel Przybilla, Jamaal Magloire and briefly LaMarcus Aldridge as a rookie. None of those guys were dominant low post players that commanded double teams whenver they got the ball in deep. The starting PGs he has been paired with have been Sebastian Telfair, Jarrett Jack and Steve Blake. None of them are particularly skilled at creating easy scoring opportunities for their teammates. I'm not sure how long Nate will stick with bringing Greg Oden and Andre Miller off the bench, but any time Martell Webster spends on the court with those two players can only help his production. Oden drawing double teams will create more open looks for Webster, and Andre Miller will do a good job of getting Webster the ball in a position to score.
Also, when Martell first joined the Blazers, they were a pretty crappy team. They won 21 games his rookie year. He was not surrounded by a lot of quality players. The players with the two highest PERs on the team that season were Ruben Patterson (17.0) and Zach Randolph (16.9) - not exactly guys who are known for making their teammates better.
Martell's second year, they added two very talented rookies and won 32 games, but they still had a lot of below average players (Jamaal Magloire, Ime Udoka, Juan Dixon, etc.) playing big minutes.
Now look at the guys Martell will be potentially playing with on the "2nd Unit" this year:
Greg Oden - 2008-09 PER = 18.1
Travis Outlaw - 2008-09 PER = 15.1
Rudy Fernandez - 2008-09 PER = 15.4
Andre Miller - 2008-09 PER = 18.6
Playing with that much talent beside him should help Martell's growth and his confidence. I know Nate rarely plays the 2nd unit as a unit during games, but those will be the guys Martell will likely be teamed with during practice (at least early in the season - according to Nat'es comments). And, since Nate almost always leaves one of Aldridge or Roy on the court with the 2nd unit, all the better.
The two things that have been holding Webster back have been a lack of confidence and not moving well without the ball. When he plays with confidence and is working to get open, he is a much better offensive player. I know the sample size is extremely small, but if you watch the replay of the 24-point quarter against Utah, you will see that Martell was constantly on the move, he was running off multiple screens, attacking the defense, drawing fouls, etc. Also, again very limited sample size, but he was also very active moving without the ball in the preseason game against Sacramento last year (yeah, I KNOW it was preseason, and yes I KNOW is was against Sacramento). He also seemd to work very well with Rudy in that game.
So, I think playing with better, more talented teammates, both in practice and in the games will help Martell's confidence. I also think playing with guys like Andre Miller and Rudy Fernandez will encourage him to be more active and move without the ball more. I think a confident, active Martell Webster coming off the bench can be a huge plus for the Blazers. I look forward to seeing how well he performs with his new teammates.
BNM