I think there could be an element that a big-budget film involving know producers, directors, actors etc. is less likely to be given the NC-17 rating than a low-budget film by unknowns, even if the content were the same as far as rating goes. That could be a factor in many NC-17 films being low-budget trash. I've no doubt there is lots of other politics involved to, with the big production companies having an easier time than smaller ones and independents. Of course, most mainstream producers these days will avoid anything close to even R to keep their films open to a wider audience. Only the big horror films are likely to come close.
I think there is little within the scope of the ratings that should mean a film is prevented from being shown in any cinemas who wish to show them, though equally there shouldn't be any expectation that they should. After all, it's the cinemas that ultimately have the legal responsibilities concerning kids getting in and the like.