I'd do it, and as others have said, it can have many longterm benifits. So TP has 4 yrs left on his contract? If so, Reke would be 25 when TP's contract expires, and still a season or two from his prime.
Now, is TP the perfect fit? No. But he's not as poor of a shooter as many make him out to be. I watch SA a fair amount, and TP is one of the better pick & roll pg's in the league. You can't do that without the ability to hit jumpers coming off a screen. He's not a great shooter, but he's above average out to about 22ft. A little short of the NBA 3. One thing he isn't though is a volume shooter, or a player who needs shots. He has taken a backseat before, and is a very calming, steadying presence on the court. He can get his shot off one on one when needed, by attacking the paint, and can create. He's also used to playing off Duncan/Ginobili. Playing without the ball won't be foreign to him.
Going back to Reke though, it's great to say with experience and time he'll learn how to run a team, and that his decision making will improve whe having the added responsibility of keeping everyone involved, and getting guys the ball where they can be effective, which a pg needs to do. It's not just setting up and offense, which Reke needs to learn. but also how to keep Cousins and Thornton involved, and getting them shots and the ball in a position where they can be effective.
I think TP would be a phenomenal teacher for Reke. Even if Reke has the ball less, and doesn't get as many opportunities(I do think he'll become more efficient with less opportunities), he'll be a better player in his prime, if he's going to have complete control over a team offense down the road. He obviously has a lot to learn, as many young players do, but he also needs someone to teach it to him. Beno is not that guy, as much as I love him. Neither is Cisco. Reke's understanding of the game would grow incredibly by spending 3-4 yrs playing with TP.
Again, not the ideal match, but definitely good enough. More importantly, the knowledge Reke would soak up, and that his mental outlook on the game would grow by leaps and bounds, I don't see how you don't pull the trigger. If you picture Reke leading a team to a championship someday, and being the floor general, he has to learn it from someone. He needs a mentor, and one which is not currently on the roster. Maybe 4 years seems too much to some. To them I say don't be foolish. Reke would be 25, and Cousins 24 when that contract expires. If TP did come over, IMO, Reke would be a better a player at 25 than if he didn't come over, and be much better prepered to lead a team deep into the playoffs.