The Bruins saw Marleau up close the last two playoffs...no thanks. He also isn't a RW which is what they need.
The Bruins don't have a spot for Thornton since he's not better than Bergeron, Krejci, or Coyle and they value Kuraly as their 4th line center. So unless SJ was going to give Joe away dirt cheap (late-round pick) and Joe was OK with being injury insurance in the press box most nights (unlikely), then it doesn't really make sense.
I was hoping the Bruins would sign Kovy for free since there was zero risk to doing so. I doubt a) Montreal would want to trade him to Boston & b) the Bruins would give up anything more than a mid-round pick.