A trade is a swap of assets, while a free agent or a draft choice is a pure addition. Getting Hayward, who is a really productive two way wing would be a nice score, and would be preferable to getting a somewhat better player like Butler or George, but giving up the farm.
Also, I don't see Ainge ever going after a center or power forward without perimeter skills. He didn't bite on Noel or Bogut, both of whom would otherwise be very nice additions. I think he sees skilled bigs as the key to our teams offensive flow.