Whatever I've gotten from theory has definitely been equalled or surpassed by reading filmmakers own writings or, in some instances, biographies - my understanding of Kurosawa, Ozu, Satyajit Ray, Fellini, Godard, Scorsese and others was greatly broadened by reading some of their own analysis of their own work, and works they cite as influences. Any of my knowledge of techniques, and of how to examine films in social context, aesthetically or technically has usually come from this kind of investigation.
I agree with you, but for a very specific reason. Most film theories, as sexy or intellectually engaging as they can be, rest on very basic and rather flimsy assumptions: films are like dreams, cinema is like a language, the camera is like the eye, the camera is like a prosthesis of the eye, and so on. If one is skeptical about film theories in general, they only really make sense as metaphors by which particular filmmakers live. So, if Eisenstein believes that the essence of cinema is montage, this may be illuminating for the viewer of Eisenstein's films, but it is not something that the viewer has to actually believe. (By the way, I realize this is an absurd reduction, but hopefully you get the point.)
However, I think it's important that one regard theory not only as a means of explicating individual works, but also as an autonomous entity that needs to be challenged and appraised on its own merits. It seems to me that, while many don't like the idea of theories of film being separate from film itself, this is precisely what makes a theory a theory: it is a (usually very broad) statement about cinema's ontology, its effects on the spectator, its place in society, and its inherent ways of conveying meaning. The practice of applying theories of film to individual works in order to "prove" something about one or the other is, it seems to me, rather more dubious, or even detrimental, than simply approaching the assumptions and generalities of a theory on its own terms and by its own logic.