Yea this is a real exception to the rule. Basically only like 5 directors working could get away with this, the economics are just not there (not to mention that it basically forces you to shoot Super 35, digital or do a blow-up of anamorphic).
Avatar was actually composed for 1.85 and manipulated for 2.39 in some theatrical releases but it wasn't actually produced for 2.39. You can get away with this to a degree on a film that is basically all CG but on a live action feature it's a little tougher (as it is because home video and international deliverables are all over the map these days you often have to protect for every imaginable situation). I watch Inception (anamorphic) on HBO where they've cropped a 1.77 image out of the center, and it's not that impressive to me because it completely ruins the composition (which is more than just what the camera is shooting, it's art direction, background actors, lighting, colors, etc).