If teams trade away their picks doesn't the rookie signing pool decrease by the amount slotted for those picks?
IMO the only reason to trade away picks for cap reasons would be if you don't have enough total cap room to sign all your picks to begin with. This year even the Colts seem to have that.
The real reason to trade picks into the future is if you have a limited number of slots where a rookie can contribute on the roster, and you see no outstanding values on your draft board. Generally the going rate for future picks is one round up for each year you trade back, and sometimes that's the best value at a particular pick.