Over the top for one major reason:
Everyone at one time or another gets their hands into something that isn't very pleasant while using the toilet. Imagine you have people at your house that used your toilet. These people could be careless friends, drunk neighbors, or tiny children. Now imagine that this person had one of those unfortunate hand soiling experiences. They reach for the TP to clean up - and you have it hanging under touching the wall behind it. The dirty hand must brush the wall to remove the TP from the roll - leaving brown, yellow, or possibly white residue behind. Now imagine you use the TP next...
Over the top is the only way to avoid this disgusting possibility.