On the one hand, SJH is correct in that Shields, by many metrics, looks to be declining right around when you might expect him to decline (as a side note, when discussing pitchers, we should never mention wins again...they are about as relevant as unicorns and posting fees for foreign players that are actually a good investment).
On the flip side, utility 3B Pablo Sandoval is giant albatross around the neck of the Boston Red Sox and everyone in baseball knows this. The Sox are going to have to pay someone something to make him and his ill advised contract go away. In short, he isn't fetching you much more than someone else's trash. As such, taking a flier on Shields and his equally odorous contract in exchange for Pablo isn't the worst trade in the world, if your upside is a guy who can eat innings at a ~3.50 FIP.
As a side note, another upside from a deal like that, if Shields is indeed cooked, is that SJH's rants will be epic and prodigious. Its a win-win for us and Best Buy who will sell many, many universal remotes.