Here's the difference betwen Peja an Dirk. Also the reason Peja is NOT a first-rate offensive weapon despite his prolificness.
I want to get Dirk a shot -- give him a scoring opportunity. I give Dirk the ball. Voila! Instant scoring opportunity. He doesn't have to be perfectly set up. Doesn't have to get the ball in one of his "spots". The whole court is his spot. And if he's not where he wants to be, he can move WITH the ball to get there.
Peja's game is different, and puts much more stress on his teammates. Oh Peja can throw up junk if you just throw him the ball just anyplace, but the percentages are NOT high. In order to use a Peja effectively, he almost has to be open BEFORE he gets the ball, or you have to run over and try to help him get open once he catches because he can't do it himself. Now the key that has made Peja more than just another Fred Hoiberg or Erik Piatkowski is that he works very hard to get open without the ball. He is essentially playing his offensive game off in space while the ball is elsewhere. By the time the ball gets to him, the shot is practically already made or not. He's just converting on his earlier effort.
But now you have certain teams who have Peja's number. They've got it figured out. And they have started playing Peja very physically without the ball. He plays his offense without the ball, and they have responded by playing defense on him without the ball. And it frustrates him, and it takes him out of rhythm. So now you have a quandry -- assuming that Peja keeps working, which he does NOT always do, do you as a team work even harder to get him shots? Or do you go to other people who aren't being as smothered? And when Peja quits working without the ball, it gets even worse -- its as if you threw the ball to Kobe (who does his offensive work WITH the ball) and he just stood there. What now?
The open Princeton-style offenses we run work very well for Peja, but NOT because they ever focus on feeding him. They work well because they focus on swinging the ball and feeding everyone, Peja included. Its a matter of feel, decision-making, who's open etc. But everybody's involved. An offense that actually was built around force-feeding Peja would be very hard to make work without grinding the whole thing to a halt. It would require far more effort to design a Peja-feeder offense than a Dirk-feeder offense, and even more still than a Kobe-feeder offense. A Kobe feeder offense looks like "throw Kobe the ball and let him bring it up court". Instant feed. A Dirk-feeder offense looks like "bring the ball up court, then throw to Dirk". Let him do the rest. A Peja-feeder offense looks like "bring the ball up court, run a play for Peja, set a staggered screen, throw ball to hopefully open Peja". MUCH more work for the team, and obviously takes away from what the other guys are doing. It only makes sense if you have a bunch of untalented saps out there who's primary skills are setting picks and playing defense. If you've got 5 talented starters, than by the time you're got all 5 of them invested in trying to set up Peja, its just not worth it. They can take care of the scoring themselves more efficiently than all of the work it takes to forcefeed Peja.
And so in the end you have a great offensive weapon in Peja who can be taken away because of his lack of ball skills and post game. We can't just throw it to Peja and say "go get 'em" without going through a tremendous amount of work to get him in the right spot and situation. And at some point, if the other team is really concentrating on making it SO hard to get it to him, you're better off looking elsewhere for an easier score.