I'm 5'9" and ride a 54cm frame. My 6' friend usually rides 58cm or 60cm frames depending on how the bike fits. If he were riding a 50cm frame, I imagine he'd be knocking his knees into the handlebars or falling over constantly due to toe overlap problems. Aside from that, it feels to me like a properly-sized frame makes it easier to put more power into the pedals. And a larger frame with a correspondingly longer wheelbase may provide more stability when touring.