Part of the reason is due to coordination problems, which I think would be reduced if the group consisted of clones of a single person with similar education and upbringing, and hence similar values/goals.

I thought of an interesting objection to this. What if the cloned agents decided that the gap between themselves and other humans was sufficiently well-defined for them to implement the coherent extrapolated volition of the clones themselves only?

http://lesswrong.com/lw/932/stupid_questions_open_thread/64r4

Of course, this problem could potentially arise even if the gap was poorly defined...