Good rationalists, taken as a group, shouldn't be systematically optimistic.

They should be if they want to win in practice, as opposed to just getting theoretically-correct answers. See, e.g., the studies referenced in Seligman's "Learned Optimism", that show optimists consistently out-perform pessimists (i.e., realists) in a wide variety of fields and endeavors.

(Of course, Seligman's definition of optimism may be different from yours.)