I don't know. I think Prince is a significant upgrade defensively (and that is where they win games anyways), and can stretch the defense as well as Gay, to open up the floor for Randolph, Conley, and Gasol. Davis also is a very valuable piece coming off the bench that they didn't have before.
You may be right defensively, but Prince at his peak wasn't the offensive player Gay is now and Prince has clearly been on the decline the last couple of seasons.
Let's put it this way. Imagine if Rondo were healthy ,and Ainge traded Pierce and the best player he got back was Prince.
When making this comparison, let's keep in mind that this year, Pierce is outscoring Gay, shooting a better percentage, getting to the line more often, outrebounding Gay, is a much better passer, and is a much better defender.
Look up the stats, the 2012-13 version of Paul Pierce is outperforming the 2012-13 version of Rudy Gay in every possible metric.
Let's also keep in mind that Gay's poor outside shooting doesn't help Gasol or Randolph at all while Pierce is a perfect complement for KG and Rondo.
I'd be a bit disappointed for that type of return for Pierce, who has a contract with a cheap buyout next season. I'd be fine with that return for Gay, who has a contract well above his actual performance.