During my CFI training, my instructor relayed a story in which he had a mid-air collision while teaching (how to teach) clearing turns to two other CFI applicants. The other aircraft was underneath them and may have climbed into them. No one in his aircraft saw the other aircraft. They did not even know they had hit another aircraft until they heard the report of another crash nearby.
My question is (in the title) why is a shallow banked clearing turn considered better than a steep one? I feel like I get a much better view of everything (above, level, and below) in the direction of the turn if it is a steeper turn. The downside is that I'm changing direction more quickly...but I think seeing other aircraft wins over that. Furthermore, large, rocking wing movements tend to make the aircraft I am in easier to see as opposed to gentle, shallow turns.