I have to disagree with the water out the ends theory. Take birch rounds for example. If left in the round the bark seals in the moisture and the end exposure isn't adequate enough to dry the wood before it rots.
Also, I find even small limb wood will retain moisture longer than larger split wood will. I have been burning silver maple cut, split, stacked Dec. '14 and the larger splits have been burning great but the smaller limb rounds have sizzled some.
I believe bark holds moisture in but when split the split face allows the water to escape quicker. I do believe the ends move water more efficiently, like a bunch of little straws together but don't think it is the only area the water escapes.
After this debate lets talk bark: stack bark side up or bark down?