There was a great article in the Hardball times Annual this year analyzing trades along with players teams let go to the FA market, and how much value the acquiring team gets out of the player
Teams know more than anyone else about the players they are letting go, Health, scouting, ect...
It was pretty in-depth so I won't go into it here, but to make a long story short, be wary of any player a team lets go via FA or trade. Even though it is applicable to position players and relievers somewhat, the largest correlation is with Starting pitching.
I realize both teams moved SP in this deal but because it makes no sense from the Rockies end I'm thinking they know something which concerns them about Hammel.