Over the past three seasons, the Mariners have pretty consistently averaged ~1440 innings pitched and ~2550 fly balls + line drives allowed.
Over the past three seasons, Jarrod Washburn has pretty consistently averaged ~2.075 fly balls + line drives per inning.
Let's say that Jarrod Washburn throws 180 innings in 2009, with the same kind of batted ball distribution. What kind of effect can we expect our outfield defense to have on his RA (relative to average)?
Chavez/Gutierrez/Ichiro, +40 (estimate): +5.86 run benefit to Washburn, reduces RA by 0.29
Chavez/Griffey (or Wlad)/Gutierrez/Ichiro, +20 (estimate): +2.93 run benefit to Washburn, reduces RA by 0.15
A lot of people out there are hoping that our defense makes Washburn look so good that, should we desire to trade him, another GM will be fooled into giving up something of value. The defense won't hurt, but for every one part defense, we're going to need five parts luck.