We have a lot of items that we both dropship and stock. If a customer has a big order of different items, we take it from our inventory instead of having 5 different shipments. Other times we may elect to dropship a whole order if the items are all from one vendor and the order has a "dropship" only item.
If an item had two warehouses - one for my In Stock and one for dropshipping, I could select the warehouse on the View Order screen and my inventory count wouldn't get all screwed up. This would also help out merchants who have different warehouses across the country with the same items stocked in each.