the only downside to such a product is reliance on the phone itself. Phones evolve so quickly, the iphone5 is already a dated design, and the 4/4s, while useable, is a 4 year old product. What happens when Apple/Samsung stops supporting the phone?
Then you have a however many thousand dollar accessory that is not compatible with whatever new phone of the week is hot.
Great idea in theory, but poor in practice. A true SDR should not be dependent on a cellphone. Now integrate an SDR with a phone over a BT or WiFi connection is another story, and alot more practical. That way it doesn't matter what phone of the week, carrier or network you are on.