The advantage I see to something like this is that it's similar to a Liberator from WWII. You have a firearm no one knows about that can be used in a emergency situation and possibly to get you a better weapon. Ideally, it never has to have a full magazine run through it.
This isn't the lower receiver that you use to build your primary rifle. This is for one of two things. 1. Use it to build the cheapest rifle you can that shoots. It can be used and tossed with little regret if need be. 2. Use it to build a cheap, disposable, lower, or possibly, put quality of you want spare parts or to swap them later. Put the upper that is exactly what you want to your specs on it. Use it to "acquire" a better lower and put the upper on it. You basically have the rifle you want and some spare parts.