I assume you mean strong outlets causing retain cycles? In my opinion this is the only argument that could be made for weak outlets and I can't think of a case where this would happen.  You would have to have an outlet going from a child to it's parent. I would consider this and edge case where you are already doing something wrong. We could explicitly state that outlets that reference a parent should be weak if we wanted.

The performance difference is another good point but I would say not worth considering.