My guess is they wanted to hedge their bets and pay for potential. Porcello is a unique case among next year's crop of FA pitchers. He's a groundball pitcher whose pitched well leading up to a career year in 2014. His peripherals suggest that he was the victim of poor defense during his tenure in Detroit, something that should be remedied with a better infield defense (though I'm rather tentative on including Bogaerts). Throw in that he's 26 going on 27, is a quality #3 (potentially #2), not to mention durable and you'd could see some team (i.e. NY) throwing bookoo money at him next winter. If last year is any indication of where Porcello is trending, this will end up as a solid deal.