If you're interested in how the pitcher-shaped sausage is made...
The Red Sox were interested in Anibal Sanchez, but they weren't going to pay $80 million for him like the Tigers did. It's also likely they weren't even going to go to $75 million, like the Cubs were willing to do. That's because Boston wasn't looking for a five-year commitment.
Source: #RedSox not willing to go beyond four years for Anibal Sanchez.— Michael Silverman (@MikeSilvermanBB) December 14, 2012
There's a reason Ryan Dempster at two years is such an attractive proposition. Pitchers are risky, inherently, and reducing that risk whenever you can is a great way to work around that. The Dodgers tried to reduce the risk of Greinke by betting on his being so good over the next three seasons that he chooses to opt-out of a long-term deal. There's still risk there -- maybe Greinke collapses, and refuses to opt-out because the markets of old beat the new reality, or maybe Clayton Kershaw bolts and causes the Dodgers to need to overpay Greinke post-opt-out in order to retain rotation relevance. But you do what you can within reason, and Boston going for two years of Dempster over five years of Sanchez fits that bill.