One would have a severe framing problem if one adopted a rule that one's estimate of X should be the average across people of their estimates, E[X]. This is because a translation of variables to F(X) might be just as natural a way to describe one's estimates, but as Eliezer points out, E[F(X)] usually differs from F(E[X]). So I think it makes more sense to average probabilities, rather than point estimates.