Well sure, if I want the absolute best low light performance, a full frame camera is going to be my best option. But if I don’t have my camera with me because it’s too bulky to carry everywhere, all of those benefits go out the door. “The best camera is the one that you have with you” or whatever that quote is, etc etc.
And while the Df is lovely and small for a full frame DSLR, it’s still REALLY HONKIN’ BIG compared to, say, the RX100 mk II 
Edit: Thanks for reminding me about dxomark though… Useful for comparison purposes.
One more edit: And there’s also the significant price difference, too. If I really wanted to go crazy, I could get the Sony RX-1
