From an engineering point of view, the external BB allows the use of larger bearings since they don't have to fit inside the hanger tube. Supposedly this permits improved wear, but I have yet to hear empirical evidence that it does.
I wonder if using larger bearings could possibly be a way to avoid immediate part replacement if the races have become pitted, since changing the bearing size should change where the bearings make contact with the races. anyway, I haven't thought about this very long, and it probably won't turn out to be a very good idea.