I know he is worth way more but a chance to win a ring might sway him to sign with Boston.
People overestimate this "chance to win a ring thing." The only players that will come to boston for cheap for a chance to win a ring are:
1. Old guys who have already made their money (like Gary Payton with the Lakers, or PJ Brown, or Cassell)
2. Role players who feel undervalued and want to pull a Posey (e.g. sign a 1 year, below market contract, so that they can get noticed on a national stage and parlay that into a big payday).
An emerging star like Josh Smith is at the point in his career where he is going to sign the biggest contract of his life. No way, in a million years, he'd accept the MLE (which, as many have posted) wouldn't matter because ATL would match that in a millisecond.