So if Auteur Theory is the investigation of a director's style/imprint across the body of their work. All directors will leave some kind of imprint of themselves in there body of work. Therefore Director = Auteur, so Auteur is a different word for director, but Auteur Theory is a useful study tool.
Not all directors make personal films in a personal style. This is the issues with Bay: while he has a clearly distinct visual style, is it actually an expression of something about the way Bay sees the world, or is it a cynical exercise in audience pandering?
Keith's articulating an extreme version of the theory on a couple of ends. Auteurists certainly don't believe writers or actors are irrelevant (see Jonathan Rosenbaum's argument that Taxi Driver
has four auteurs
: Scorsese, Schrader, DeNiro and Bernard Herrmann). Nor would they argue that every director is an auteur. One of the main games auteurists play is sorting out the hack directors from the competent but impersonal directors (metteurs en scene) from those directors with a strong personal vision/style/obsession.
Also, the theory is not the end of the conversation. It functions as an organizing principle, but is not, in itself, criticism. The important thing about Andrew Sarris's The American Cinema
is its organization by director. But the great thing is that Sarris uses the organization to discuss each directors work in the context of all their other work, in terms of both visual style and thematic concerns.