Murphy wasn't going to get another shot at the NHL in MN. GMPF just added Hunt and Bitetto (guys he had in NSH) to go with Prosser in the 6D/pressbox rotation.
I see it more as a the GM doing right by a player, where maybe another team gives him a chance MN can't/won't in the NHL. They did the same thing with Reilly last year by sending him to MTL for almost nothing. I don't see Murphy (UFA after the season) being in the team's long term plans. In return they got a RFA, who they can get a look at and decide to keep him or not after the season.
At worst the trade makes the Wild's AHL a little worse.