New York is a mess with Amare + Anthony, lots of talk how they are a bad combo. Trading for Pierce makes them better now (more of an all-around player, can play alongside Stoudemire), and gives them a lot more cap flexibility moving forward (and New York CAN make use of cap room). They get back a legitimate star before things get worse.
Boston gets a top-25 player with top-10 ability, 7 years younger than Pierce. Look at the situations 'Melo has been in: early unwinnable comparisons with LeBron, playing alongside Iverson, traded to New York with unrealistic expectations (Amare, Chandler and a gutted roster with no point guard trying to play in a D'Antoni offense). I'm not saying he's free of blame, but he's a hell of a "buy low" target.
Personally, I'd add draft picks to the deal on our side but obviously would rather not.
If we had Rondo, Anthony AND cap room, do we get Howard? That's a stud trio in their prime for the next 5+ years.