Much like Carmelo did, Smith could "choose" where he wants to go by letting all other suitors (the Bucks, Suns, Sixers, Nets of the world) know that he only has eyes for Boston.
That also gives us a lot more leverage with the Hawks to get him for OUR price (i.e. not Pierce).
Some team could put the best package in the world together, and it won't matter if Smith is explicit that he won't sign there. No one will give up a bunch of talent for a 3 month rental.