We're all taught about the limitations of the mark one eyeball, so there's no point in pretending it's foolproof. I've often been informed of traffic while flying, and not been able to see it, even though I knew where to look. However, if I hear "final" when I'm about to be in the same place, I'll know to slow up my approach. That's why I think we'd all be safer if we used radios in the circuit, even where it's legal not to. The cost of a handheld is small beer compared with running an aircraft, and could be a lifesaver.