Peter,
One issue that I observed with our users while working on this issue, was a bad habit of using the "Finish" button or "Ctrl-F". They would often use the Finish button even though they did not make a change to the purchase order.
Since the Finish functionality is "Save" and "new', this process saves a revision even though the users did not make any changes. On a large purchase order, it is not difficult to hit 99 revisions when users are misusing the Finish button.
IMHO, this issue is definately a defect. However, a little retraining has gone a long way to alleviating our issue with the 99 PO revision defect/design flaw.
We also make an effort to minimize the number of line items on a purchase order.
Good luck
Steve