I seem to recall that it's the basis for expecting humanity's extrapolated volition to actually cohere.

This whole area isn't covered all that well (as Wei noted). I assumed that CEV would rely on solving an implicit cooperation problem between conflicting moral systems. It doesn't appear at all unlikely to me that some people are intrinsically selfish to some degree and their extrapolated volitions would be quite different.

Note that I'm not denying that some people present (or usually just assume) the thesis you present. I'm just glad that there are usually others who argue against it!