Its a great thing for a couple of reasons.
1. It keeps Punk relevant while he's not able to wrestle. Unless you're at the top of the card(and Punk hasn't been for a year now), an injury often times leaves the wrestler as an afterthought, at least in creative's eyes. Rey Mysterio returned at Summerslam 07, less then a year after winning the World Championship, and toiled in utter futility feuding with the likes of Chavo, MVP, and Finlay before being drafted to Raw to feud with Kane. He wasn't re-established until the end of his Raw run, in 2009. So Punk on commentary will keep him fresh in the eyes of viewers and relevant.
2. Hopefully, it will allow Cole to back off his heelish antics some, as Punk can take over the role of needling wrestlers. Allow cole to continue to play the Daniel Bryan/Kaval/Internet darling hater and Miz's cheerleader, but leave it at that. Punk's knowledge of the wrestling business and his obvious passion for it should allow him to be insightful as well as heel it, allowing Cole to be more of the tweener play by play announcer who can accentuate wrestler's positives rather then ripping into every face every chance he gets.
3. He's really darn funny. From his reference to Alex Riley's DUI, chanting along "Cena sucks", or basically saying what we all think of the Divas, Punk's shown that his night on NXT wasn't just a one hit wonder. We all knew he was a good talker, but his biting sarcasm is a refreshing change from Cole and Lawler.
So yes, I think Punk joining the Raw broadcast team is a great idea.