I honestly have no idea whether or not Tim Cahill running as an independent will hurt or help Deval Patrick. Some seem to think it helps him (assuming Baker's in), others seem to like Cahill's chances. Sab just seems to think everyone loses.
I could think of reasons why it could hurt and help -- this far out, it's honestly pure conjecture. We have no idea how Cahill will act as a candidate (I can't imagine competently), nor what his impact will be on the race. We don't know if continual chicken-candidate Charlie will actually grow a spine and run this time, or if he'll be a competent candidate should he decide to do so.
Here's what I know, though. The Democratic Party stands for something. It takes courage and ideas to win a primary. It's very easy to turn away from courage and ideas; it's very hard to win a General Election without the backing of the Democratic Party. We don't know if Cahill will help Deval win reelection or help get the Republican in office, but at the very least we'll have a new state Treasurer. Perhaps we'll have someone who knows what they're doing.