You're trying to compare, even-up, the +/- ratings of individual players on a top quality 78 point team to the +/- ratings of individual players on a middling and developing 64 point team. Here's the problem with that attempt:
Seguin is one of only 3 players on his entire team to have a double digit positive +/-. The other two are Nichushkin (18 y/o RW) and Benn (D). Boston has 14 players with a +10 or better =/-, and only 5 players with a negative tally. Dallas only has 14 players at +1 or better, and 14 with a 0 or worse =/- rating.
As for Boston being built to win now, that's simply not true. Most of the team is 30 or younger. That team was deliberately built to extend its window. The only currently active players who are over 30:
Kelly
Iginla
Thornton
Chara
And, yeah, when I'm watching a team that struggles to score against good defensive teams, I get to thinking that keeping scorers/playmakers is a good thing.