Why were the Boston Red Sox so intent on clearing Marco Scutaro's contract from the books?

According to Alex Speier, it was all about the Competitive Balance Tax (CBT) ... also known as the luxury tax. The actual math is sort of complicated and Speier's a mensch for going through all of it, but here's the nut:

When the Sox exercised the shortstop’s $6 million option for 2012, it transformed his deal into a three-year, $17 million contract. That, in turn, meant that for the coming season, he would have counted for not just his $6 million salary for luxury tax purposes, but instead $7.67 million (the difference between the $17 million he’d make and the $9.33 million for which he’d counted in 2010 and 2011).

In dealing Scutaro to the Rockies, the Red Sox shed $7.67 million in CBT payroll for the coming year. For the Rockies, that’s an irrelevant consideration, since they’re not bumping up against the luxury tax threshold. But for the Sox, who are nearing the $178 million threshold, that CBT figure is a major consideration.

So it's about a) avoiding the Competitive Balance Tax, and b) signing Roy Oswalt. But what if the Red Sox don't wind up signing Roy Oswalt? Or someone like him? (By the way, there aren't many like him.)

Once we learned the Sox were desperate to keep within a certain payroll -- for whatever reason, and however surprisingly -- there remained yet another question, which still confounds me ... They couldn't get more for Scutaro than Clayton Mortensen?

Maybe he'll turn into a strike-throwing machine while remaining a groundball-producing machine, and the Red Sox will have another Derek Lowe on their hands. Maybe the Red Sox' internal metrics show Scutaro as a sub-par defensive shortstop. But while I'm grateful to Alex Speier for providing a piece of the puzzle, there's still some stuff about this deal that I can't figure.