There's interesting social psychology to back that up: status ambiguity as the fastest way to endear yourself to the new group. Representing higher status than the entire group marks you as a threat. Representing lower status than the entire group marks you as a waste of time. Projecting a fuzzy medium somewhere below the top people and above the worst endears you to everyone.
Very true, but there are also arguments about why you should always surround yourself with people that are smarter than you for both personal growth and bottom line results. Especially in the fast paced startup culture, you need the smartest and fastest people around.