sstorkel
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.
I have to agree with sstorkel. My wife is 5' 3" and rides a 50 cm frame. I am a little over 5'11", and when I try riding it on a short test ride after maintenance, I feel like my knees are going to hit the bars. I don't think I could ever get the seat high enough to ride comfortably or efficiently. My current bikes have 58 cm frames, but I was fitted and rode quite comfortably a 62 cm bike when I was younger. I prefer the slightly smaller 58 cm frame size for the type of riding I do today.
I believe bike fit is probably one of the most important factors in comfort and efficiency. Fit really comes into play when riding 50-60 miles a day on consecutive days for long periods of time. My wife averaged 50+ miles a day for 74 consecutive days on a professionally fitted bike and had no issues. She was fitted by a physical therapist who specialized in bike fitting because she has leg length differences and a torn ACL. Fit really makes a difference.
Having said all that--If the bike is comfortable for you and you enjoy riding it, ride it.