I wouldn't call the Rangers retarded. How sure of a thing is Volquez? Is his potential as a number two or three starter worth more than Hamilton's potential a .300/.400/.500/.900 play in CF?
Hamilton hit 19 homeruns in his first 300 AB's in the major leagues. He's an awfully good talent, and I wouldn't be surprised if he hits 30 HR's, if he gets to play everyday.
Take into account that the Rangers were going to go with Marlon Byrd in CF, and David Murphy in RF. I think it's safe to say that Hamilton is, at least, a small upgrade over those two