As with the Nick Blackburn contract, the Twins are basically buying out his arbitration years. I think Span might be free-agent eligible for the option season.
There's a line of thought that says: The player is under team control during the arbitration years anyway, so why assume the risk of guaranteeing the money?
The answer is cost certainty – which matters because of (you guessed it) the looming expense of the Joe Mauer contract extension.
The Twins appear to be lining up exact costs for their core for the next five seasons. Scott Baker, Blackburn, Justin Morneau, now Span. Michael Cuddyer is signed through 2011; the Twins picked up his option for 2011 during the winter.
A few other guys might be trickier. Jason Kubel signed a two-year deal last winter; he might still be arbitration-eligible after this season. Putting a value on Kevin Slowey and Francisco Liriano after their injuries doesn't figure to be easy.