Let me play devil's advocate.
1) Howard's extension
After a year in a top-notch organization, trophy(?), old teammates (Lee, Bass, and maybe Pietrus), and max salary rights, Celts have a good shot at retaining Howard.
2) Obtaining Howard extends the Celts window of opportunity for ten years, and keeps Boston as a desirable landing spot for veteran free agents looking for a title and willing to take a pay cut.
3) The Celts are one significant injury to Garnett from being non-competitors for the trophy.
4) Potential Outgoing Assets
A) Bradley - shoulders may not hold up long-term given his style of play, and he will always have match-up issues with tall guards like Joe Johnson or Kobe.
B) Sullinger - great rebounder, but will be limited in post on offense by height, and is not a shot-blocker on defense; plus he has been already shown to have very poor foot speed playing against faster power forwards who can put the ball on the floor (LeBron would have a field day). Players are going around him with ease when he is drawn out to the perimeter, and that's just the summer league.
C) Melo - may only become very useful when Garnett on last legs; may not be a significant contributor for immediate push for trophy
5) Pierce and Garnett cannot carry this team through 4 exhausting playoff rounds any more ... they will have off-nights due to age, especially on offense
6) Synergy - Players like Garnett and Howard cover up the defensive deficiencies of players like Bass and Sullinger
7) Doc leans to winning games by defense. Howard and Garnett create a formidable interior defense for the final minutes, and Garnett can leave the lane to cover Chris Bosh in the corner if there is another dominant rebounder on the floor.

Keeps Howard out of LA and lets Boston match-up height-wise with Bynum/Gasol combination. May force Miami to keep two big men on the floor at the end of games, keeping Lebron at small forward in final minutes, where he is less effective and can be covered by a quicker player. Miami coach will have an interesting choice to make ... attack with Lebron at power forward and another shooter on the floor, or beef up the defense.