It's not just so it can be used as an avatar. It's done with the implication that the Na'vi will not notice or will be more sympathetic to somebody pretending to be of their race. I found it insulting. I'm part Native American and part Mexican, and if somebody showed up dressed up in traditional ceremonial clothing trying to get my land, you can bet your ass I'd be insulted.
But let's get back on topic. I'm sure Disney isn't that concerned about the sociological implications of a film. If they were, then we wouldn't hqve Song of the South.
And yes I knew that they knew, my point was that they didn't care at all that people were creating artificial bodies and thought they could walk among them. To me, I would be mad. I'd want you to come to me in your real body. Disrespect.