Different mindsets. IMO I think you build around your best players, if its a goalie then so be it. I just dont think Rask is their best player, or even close. I think you could put many goalies in Boston and make them an all-star.
IMO Rask over Seguin is a pretty dumb move that will hurt Boston 3-4 years from now.
I'm not sure why you are looking at this as a direct Rask -vs- Seguin thing from the Bruins pov. Having read Chia and Neely's comments on Seguin, its fairly obvious moving Seguin was about concerns they had about him with his big deal starting next season. Neely said if they couldn't get what they wanted for him they were going to move forward with Seguin on the roster. Then work with him on becoming a better professional among other things. I'm not saying Rask's contract has no impact but its not quite the either/or decision you're making it out to be.
I also don't see how the Bruins are going to regret this move in 3-4 years. The Bruins added the best player in the deal right now (Eriksson) whose game will be a perfect fit in Boston and he's only 27 with 3 years left on his deal. They also added 3 young players/prospects as well. Plus if Seguin puts it together in Dallas as a center and becomes the #1 center he has the potential to be. It still doesn't really matter because he never would get that chance in the next few years in Boston with Begeron/Krecji there. If anything this deal is a win/win for both the franchises involved and key players involved (Seguin/Eriksson).