Seguin is great, I loved cheering for him, but the Bruins got equal value in return, plus prospects. And more importantly they saved a bunch of money that allowed them to sign Iginla. Without Iginla you could argue that this isn't the best team in the league. Some stats beyond simple goals and assists:
Players Boston received = Smith and Eriksson
Players Dallas received = Seguin and Peverley
Bos points per minute of playing time = 0.041
Dal points per minute of playing time = 0.045
--> not bad at all, especially with Eriksson having a rough season. But what's even more important with the cap:
Bos $$$ per point scored (millions): $0.058
Dal $$$ per point scored (millions): $0.078
This second number is very important. When combined with the first set, we see that yes, Seguin scored more than Smith or Eriksson. But he also gets more ice time and more ice time than any forward on the Boston roster. In terms of getting results per minute of playing time, the new Boston forwards are almost as good as the old ones. But more importantly, they do so at a much, much lower price. Objectively speaking, you would be paying Seguin more money to get the exact same results.
There are additional advantages to having the Smith/Eriksson combo as well. 1. They're two players. Eriksson missed quite a bit of time this year, but the Bruins didn't miss a beat in large part because they also have Smith. If Seguin got injured Dallas would be in big trouble. This is especially important in the playoffs, which are often a war of attrition. 2. They are wings, and wings that can play either side. The Bruins have 2 (now 3?!?) outstanding centers. Many great forwards like playing center. Smith and Eriksson don't. They like wing, and this gives the Bruins 3 solid lines with players playing where they love. For Seguin to thrive in Boston he would have needed to displace either Krejci or Bergeron, and that would have meant a couple years of growing pains while his defensive game matured. That could have cost a few Stanley cups. 3. Smith and Eriksson are solid, all around players. Smith is slightly less physically skilled than Seguin, but I would argue his mental game is better. He also competes hard in all areas. Eriksson, also, is a fine all-around talent that plays all 3 zones and penalty kills. Seguin is awesome, but I would argue the only way he is truly better than the new players in in goal-scoring and top speed. In other areas of the game, the new Bruins are as good or better.
In the end I think Boston is much better heading into the playoffs with Iginla-Smith-Eriksson on the RW than they would have been with Seguin-Peverley-Caron?!?