In my opinion, I just don't think the value is there for Boston to trade Krejci for Shattenkirk even if Shattenkirk is the best defenseman currently available on the market. In my opinion the Bruins are a worse team losing Krejci and adding Shattenkirk rather than keeping Krejci and letting the young defensemen they currently have develop. That may sound silly, but I just don't think that trade while subtracting Krejci is going to get the Bruins near a Stanley Cup this year or in a couple of years. In the next two years we will most likely see Chara retire, and it will leave Shattenkirk without that defensive anchor to pair him with that will cover his defensive inconsistencies. What the Bruins will have is a defenseman locked in for 5 more years at $7M that may be solid defensively, but won't be able to carry a 1st-pairing just like the Bruins most likely won't count on Krug to do so when Chara retires.
Other Bruins fans and our front office may see it differently, but I would rather have the defensemen we currently have in our system and in pros develop in that 2-year span rather than trying to hot-fix a problem for the short term.
The Bruins did a big no-no last season by punishing their young defensemen for making mistakes on defense. If the Bruins went in this year with guys like Morrow and Colin Miller having a full year of NHL-experience, they would be in a better situation right now. I don't disagree with the point that the Bruins should be adding a defenseman this offseason, but I don't think giving up Krejci for Shattenkirk is the right move. The Bruins were a top 5 team in offense last season. In my opinion, the Bruins need a mobile defensive defenseman not an offensive defenseman where defense isn't his strong suit.
The Bruins promoted Cassidy to help improve the fluidity out of their own zone. I had a conversation with a fellow Bruins fan about how the Bruins can succeed on defense, in regards to breaking out of their own zone, without adding a player that has that ability. If you would like, you can read that conversation in these two posts:
http://hfboards.mandatory.com/showpost.php?p=120663027&postcount=273
http://hfboards.mandatory.com/showpost.php?p=120665155&postcount=276