My guess would be if someone swings a sword at you in a downward fashion it would slide or deflect off a round shield whereas it might just pull you down/knock the shield out of your arm if it hit the top of a square shield.
Thanks. Based on some shield images (http://images.google.com/images?sourceid=mozclient&ie=UT...) it looks like there are non-circular shapes which provide at least some of those benefits as well as possibly improved coverage of an elongated object, like a soldier.
