If he's willing to come here for the minimum and not be guaranteed minutes, it's a no brainer for both sides.
For Peja, it's his best chance at a title and some meaningful minutes. Kobe-Artest-Odom and Carter-Pietrus-Rashard Lewis make the Magic and Lakers unlikely. Here, he can spell Ray and PP and play with either while the other sits. In addition, he is not a bad pairing with Marquis, especially with Nate.
I think the Celtics help/team D will help mitigate, although not totally, his most glaring weakness.
He costs us nothing and if he is such a liability on D that he's not worth the aggravation, just cut his ass. There's very little downside here for us and a whole lot of upside given his shooting talent.