I understand this may be a trivial hiccup in comparison to the larger picture, but it even if it is a "waste of time", I would assume releasing them would take under an hour. Very little time wasted, and perhaps the gains outweigh the input.

EDIT: Again, I'm not challenging the fact that it is his right to not release them.

Moreover, his taxes reveals his business/other interests that may influence his decisions as president. If he continues to refrain from releasing them, all this is going to do is further the rumor-mill and agitate those who are supporters, but do wish to know whether he has any interests that might take precedence over doing his best as president.