Making a trade because you think you need to make a trade is how you wind up on the wrong end of a deal like the Thornton trade to San Jose.
Boston should have been able to get more, but It gave the Bruins the cap space to go out and sign Chara, and Savard.
There's an opportunity cost to be considered when holding on to a high salary player. Without moving Thornton, there is no way Chara is a Bruin, and probably not Savard either. One was a huge part of their SC, the others career shortened by a cheap shot artist, but still a great player.
When Boston accepted the package they did, I'm sure a big part of it was because of the money it opened up, which needs to be considered.
edit: Obviously this is on topic because of all the money Mullen for Allen opens up