I don't necessarily agree with this statement - if I know my best friend inside out we will have a super large room for flexibility, and that often prevents conflicts.
My practical experience is that there is no way of separating the two things, friendship and co-worker. And I believe there is more to lose than to gain in this arrangement. When a problem comes up I rather not argue with my friend (sorry, co-worker) about how my position is right and his is wrong.