I disagree; unless you're shooting a cartoon, everything should be as realistic and beleivable as possible. And everything in the movie should strive to be a work of art in itself.
I agree with this filmmaker, "The goal of special effects shouldn't necessarily be to look realistic, they should be works of art themselves and help create a mood or tell a story." Look at Aleksandr Petrov's Old Man and the Sea vs the B&W 1958 photograph version with Spencer Tracey vs some not yet made 3D surround sound version vs Hemingway's book. Which is the most "realistic" version? Which is the most "artistic?" CGI, green screen, 3D, multichannel sound is not the end-all in movie creativity. The art of film is in showing us a particular point of view, compressing time and space, focusing on aspects of visual space which lend themselves to moving the story forward. I'd highly recommend looking at The story of Film:An Odyssey which shows that early attempts at film didn't understand that the art of film is in what it doesn't show. If you don't believe me, set up a 3D HDR camera in an apartment and film daily life 24 x 7 x 365 and see if anyone wants to watch it.