to think that Murphy will not play a lot for Boston is a ridiculous statement. Jermaine will not be healthy til next year. Shaq plays limited minutes even when healthy. That leaves Murphy and Nenad. Murphy's defense may not be all that good in the past but how long do you expect him to be poor at defense on this squad that emphasizes defense.
Murphy (hopefully) will be with us next year when BBD decided to test free agent waters again and is now unrestricted.
Relative to Miami, he will not play a lot here. He will have double the minutes opportunity there.
You could make the argument that is more valuable for him to play on a team that is more of a sure thing for an extended playoff run.