Well, I suppose we can reliably expect that there are not enough people like Bob, and me getting tortured removes much more utility from me than it gives Bob, but that's missing the point.

Imagine yourself in a world in which the vast majority of people want to subject a certain minority group to eternal torture. The majority who want that minority group to be tortured is so vast that an FAI with an average human utility function-based CEV would be likely to subject the members of that minority group to eternal torture. You have the ability to create an FAI with a CEV based off of the average human utility function, with your personal utility function, or not at all. What do you do?