Well for one, the garbage contracts we'd be taking back would be Darko, Pekovic, and Ridnour. That puts the C's on the line for 12 million dollars in cap room for the next 3 years with mediocre talent (and that's kind of being generous).
Secondly, Minnesota does this deal knowing full well the Ray Allen cap relief they'd get would only land second tier FA's they'd have to over-pay for. They won't be a winner and they won't be a big market team anymore than they are today. Right back to the lottery with no sure thing for the future. They might as well advertise "Large contract for middling journeyman" on craigslist for all the good it will do them.
Thirdly, the Celtics break a faithful agreement you understand they entered into with Ray Allen the day he exercised his option. Agents don't forget that, and agents have as much sway in the NBA as any player.
I hope Derrick Williams (a guy who be competing with not one, not two, but three established minute hogging players on our roster (Pierce, Garnett, and Green) is worth it.
EDIT: Now were I Minnesota, (and I'm obviously not..that's a state and I'm just a guy), I'd trade Darko, Pekovic, Ridnour, and the #2 for McGee, the #6, and maybe try to slide Nick Young in there too and be done with it.