I don't know anybody who went to see Avatar because they thought it would be a good movie. I also don't know anybody who went to see the 2D version, despite it playing at the same time and a few bucks cheaper. Everybody went because they wanted to see what could be done with the cutting edge in CGI technology plus insane amounts of money and to see what this 3D cinema thing was all about. I have as little evidence as you do, but I imagine that skipping the 3D version wouldn't have gotten the film anywhere near as much hype (and thus money).