That even if the whole of the thing is just a dream it should not change the way we view the film because all films are collective dreams.

To quote (some dude on the internet):

"Every single moment of Inception is a dream. I think that in a couple of years this will become the accepted reading of the film, and differing interpretations will have to be skillfully argued to be even remotely considered. The film makes this clear, and it never holds back the truth from audiences. Some find this idea to be narratively repugnant, since they think that a movie where everything is a dream is a movie without stakes, a movie where the audience is wasting their time.

Except that this is exactly what Nolan is arguing against. The film is a metaphor for the way that Nolan as a director works, and what he's ultimately saying is that the catharsis found in a dream is as real as the catharsis found in a movie is as real as the catharsis found in life. Inception is about making movies, and cinema is the shared dream that truly interests the director."