In addition to Ray Allen being a future Hall of Famer, he is pretty savvy politician who cares immensely about his legacy and the jersey that will be inducted into the Hall of Fame. Secondly, if he signs with the Heat, it simply means he thinks they are better than Boston, particularly, if he takes half the money to do it. I don't think he can stand the sight of being booed in the Garden (a big piece of his legacy); and he will be booed.
His legacy will be enhanced more if he won another one in Boston versus winning another one in Miami. Despite all the talk about this being Rondo's team now this era will be remembered more as the Big Three's (Garnett, Pierce, and Allen) and the team sacrifice they made versus the other superstars in the league. Let's say Paul, KG, and Ray hang it up in two years and ascend to the Hall in the same class (with Doc Rivers presenting them); that is infinitely more valuable than if he played the last two years as the Heat's sniper, where Lebron and Wade take the oxygen out of the room.
Finally, let's remember Ray Allen's commentary on David West when he decided to sign with the Pacers instead of the Celtics. Ubuntu is what these guys stand and it is part of their legacy and I think Ray Allen knows that.