Spoelstra has made quite a few comments about Mike Miller's ball-handling ability and his ability to the run the point guard position too. Citing his good play in the backup PG role in Memphis a few years back (which I cannot remember but appearantly it happened in the latter months of one of those seasons).
Anyway, those three are going to be so interchangeable offensively that you could call anyone of them (Wade, LeBron, M.Miller) the point guard when they on the floor together. They'll share the PG duties. They'll change up who brings the ball upcourt, who initiates the offense, who is the playmaker or shot creator.
By having Miller initiate the offense, it'll be easier for LeBron and Wade to concentrate on their scoring. Be score first pass second players. Also allow them to catch the ball closer to the basket, or away from the defense, not have to face the defense from 25+ feet and try to create something. Keep some more player movement offensively.
Anyway, names, roles, positions ... it's fluid.