There are so many applications for this one. The one that comes to mind for me is if a customer makes a purchase and lets us know they'd like to add a recurring order to the original order. Why should they have to go back through the shopping cart again?
We usually do these in Authorize.net, but then our 1ShoppingCart data doesn't reflect the partial refunds. This should be a high priority.
I love this idea. I've wanted something like that in 1SC for a long time. What a great way to make new sales and get new customers without creating a new product, especially at the holidays!