Consider the following sentence:
The independent filmmaker Ira Sachs’s drama “Keep the Lights On,” which opened on Friday, is neither an especially good movie nor an especially bad one, but it is an exemplary one.
I am confused about the comma that precedes "but." Is "It is an exemplary one" an independent clause? The rule of a thumb I use is a can-it-stand-by-itself test; the sentence above sounds kind of awkward by itself, though I guess it could work. If it's not an independent clause, then why the comma?