I think all you have to know about this deal is, over the past three seasons _ since Matthews enjoyed a breakthrough 2006 that quite likely included HGH usage and resulted in a huge contract - Matthews has put up a combined rWAR of -.6. In other words, during his time with the Angels, when he was making $10 million a season, he was worse than your standard, Quadruple-A player.
Brian Stokes? He has a .1 rWAR in the same period. He's a tick better than your Quadruple-A player. And just counting, 2008 and 2009? Stokes' figure goes up to .9. Matthews' goes down to -2.2, thanks to apparently deteriorating defensive skills.
Omar MInaya mentioned the "change of scenery" thing, and hey, stranger things have happened. Matthews clearly grew deterred after the Angels, deterred by Matthews' poor 2007, relegated him to fourth-outfielder status by signing Torii Hunter.
But $1 million per season is not worth a "change of scenery" risk. Not when Matthews, for the most part, hasn't been a very good player.
Very bad trade.