Kessel's departure, I think, was reflective of the front office's belief that the Bruins overachieved last year and weren't likely to be a Cup finalist this year either...I'm of the mind that they're looking a few years down the road.
The way this team is playing, Kessel being in the mix wouldn't make all that much difference I don't think, although the Savard and Lucic injuries clearly sapped that B's offensive depth and perhaps. With them playing, Krejci, Ryder, Sturm et al might find more favorable matchups which they can exploit.