It is not clear what you mean by an "unnecessary inventor".
Inventorship is important in the U.S. Anyone who makes a conceptual contribution that ends up in a claim is a proper inventor. It is not always easy to decide who is an inventor. One of the reasons inventorship is important is that, absent assignments, each inventor has an undivided, separate right to the invention with no need to coordinate or split money with the others.
It is also important becasue the validity of the patent can be attacked for incorrect inventorship. Until the AIA law went into effect, patents could be invalidated for incorrect inventorship because to correct inventorship required sworn statements under penalty of perjury from various people that the error in naming inventors was made "without deceptive intent".
Under the AIA, that requirement is gone so the list of inventors can be fixed, even in the middle of an infringement lawsuit without worrying about perjury.