Golson looks like Jeff Jackson Part II. The power the FO thought he would develop never has, his plate discipline is awful, and his baserunning blunders after he was called up in Sept. did not endear him to the major-league coaching staff. He only has value as a CF, and he was blocked there by Victorino and would have been for a long time. He would have been a serious negative as a corner outfielder.
So he may have value someday, but he was not likely to have value for the Phillies anytime soon.
Mayberry, on the other hand, has more power, somewhat better plate discipline (though it hasn't been great recently), and hits lefties very well. He could have value to the Phillies as soon as next year as an RH bat off the bench or maybe even as the RH side of a platoon in LF. He does have some holes in his swing, but the organization fixed those of his Stanford teammate Michael Taylor, and maybe it can fix Mayberry's too.
So I like the deal.