I think the premise of the original post is that, given Ainge's track record, he's earned the benefit of the doubt with the Perk trade. And I agree with that, even though I'm still not sure how I feel about it.
Like all GM's, he certainly has made some mistakes though. The Gabe Pruitt and JR Giddens picks were both terrible. I realize they were both late first round picks, but I think it's fair to expect a guy taken at that point in the draft to become at least a legit rotation player. Neither one did.
As for the first Walker trade, I wasn't against moving Walker, but like someone else mentioned, Raef and Jiri were both busts. Jiri was horrible (although we did fleece Cleveland by getting a 1st for him), and Raef was one of the softest big men of all time. Off the top of my head, I can't think of a Celtic player since Ainge arrived that I've despised more than Raef. We definitely could have gotten better value for Walker.