Matt - yes, we also have the actual Sandbox system where no transports are moved either in our out.
In this case we are dealing rather with temporary DEV systems. Personally I'm not fond of this setup at all and it causes a lot of headaches. But it's not my decision, unfortunately, and we have to accommodate business, not the other way around (for some strange reason 
).
Agree that this could make an interesting architecture level discussion.