There were probably other options. Like, for instance:
Keep Perk, trade Erden/Luke for Anthony Parker, add Brewer, add Murphy
I really don't think Danny or Doc wanted a repeat of last year's wing situation, where we had two wing choices and neither of them were two-way players. We could go defense-only with Tony Allen or offense-only (and not that much of it) with Michael Finley. Now Parker is a lot better than Finley was last year, but he does fit into the category of mostly one-way player who doesn't have a tremendous impact on his one-way (that and Brewer isn't as good as TA).
Jeff Green gives them the first two-way wing they've had since Posey.
I think a better counter-scenario would be something like Big Baby and Marquis Daniels to Phoenix for Pietrus. That way we keep Perk for the stretch run, we use Murphy to fill-in Baby's role and we get our 2-way wing. Then we pick up Leon Powe as the 3rd string PF.
I would have viewed that as a very solid, almost risk-free move. Less dependent on JO and Shaq, less chemistry fallout.
Still, I can't fault Ainge for getting a more valuable wing in Green, decent Shaq/JO insurance in Krstic, a Clippers pick as compensation for Perk's inevitable departure, and keeping the lesser but still durable, versatile Big Baby over the one-dimensional, injury-prone Perk.