This is the biggest reason. Dickey is only owed $5 million this year. Very cheap. Better than Shields as teams are starting to realize that, if you believe you can compete that year, you have to go all out. Acquiring an ace for $5million allows a team flexibility to sign another top flight player financially.
Also, they see how Dickey was a sensation in NY. Fans came to see a losing ballclub just to watch Dickey. You can't buy that draw, especially in an era where the sellout in baseball is becoming rather rare. Also, a lesser talked of thing but still great, players love Dickey and he's a loyal guy. Would sign extension with a team if they give him respect.
All this age talk is legit; 38 years old is old. But you also have to look at depreciation of all talent, regardless of age. Who says Shields or Grienke won't tank out after transferring teams? Look at Barry Zito and that dilemma. Dickey may outperform both Shields and Grienke over a course of a couple years, regardless of age.
You can't honestly believe that, right?
According to this, Barnes is #2.