There are variant cuts of The Terminator, Aliens, The Abyss, T2 etc out already but they haven't supplanted the theatrical versions and none of them as far as I know have ever had a theatrical release, they certainly haven't replaced the original cuts as the only possible means of a theatrical showing.

The real test of Cameron's words will come when the DVD/Blu Ray come out and if Avatar gets further theatrical runs.

If this longer version is the only version available to own and see for further generations then yes he is being a knob for calling out Lucas.

If both versions are available and serve both functions then there would be no difference between what he has done and what Peter Jackson did with the extended versions of LOTR or what Spielberg did with ET.