i think the logic is "we're good at getting a few productive years out of discontented players, then dump them at the first sign of trouble. so here's haynesworth available for basically nothing, and we've got to spend all this cash. so let's go for it, and we can always cut him at camp if we need to."
makes sense in that respect, and belichick does have a history with these kinds of players--corey dillon, randy moss, rodney harrison, etc. but the teams they came on to had really established veteran leadership. not sure the current roster does, other than brady.
but with haynesworth, there's also the question: is he even good anymore?