I can't speak for the Bruins but the reason I suggested the deal was because I thought Colborne could be a serviceable (albeit temporary) 3rd line center for us.
Colborne and Hayes have similar stat lines and have been used in basically the same role this season, but last year Colborne had 19 goals and 44 points in CGY, he was also 54.8% at the dot. Having seen both of them up close, Colborne is MUCH more skilled than Hayes.
I'm wondering if Colborne just isn't a fit in COL. They're committed to Soderberg as their 3c, so Colborne has played mostly RW (his off-wing). They're also going to trade Iginla so adding a RW in Hayes fills a spot for them and maybe he plays better out from under the pressure and stress he deals with here. For Boston, they could try Colborne at center and see if he can find his game again and maybe give Spooner someone more skilled to play with.
Worst case, it's garbage for garbage and neither team is any better/worse off. Best case, the change of scenery and role does them both some good.