From my point of view, it would be a great way to reduce the "billing-departments" cost in hours and fees (paypal, credit card company etc)..

If we are talking about a big amount of clients, you could easily "save" around 100 to 200 $ each month on management...

I would never do it myself, simply because you may lose sales for the same amount that you save, because people don't wanne take the chance...