I find it hard to believe that they don't work at all. Simple physics would suggest they would be good at diffusion based on them providing more surface area. I'll agree, they probably could add distortion based on rattle, and I doubt they help with negative reflection as the sound likely won't pass through them very well, but as far as high frequency diffusion - they would work on a basic level. Now I have to egg-carton up my studio and get out my white noise tapes and make some recordings...
on a more practical note: you can find foamvar insulation very cheap. It's usually pink or tan, but it is such a dense foam that it will definately dampen a room. I used to have my entire studio covered in it with spacers to give it an angle at every perferation... when from a slap back echo to a dead room in a hurry. Dirt cheap too. FYI.