Not to mention you would need to attach/detach the lens, connect Bluetooth, and likely open an app if you want a viewfinder. That's a lot of steps compared to just flipping on a power button.
Aaand, you just lost the money-shot.....
You people don't get it, right? Well if you ever used RX100 you know that the pictures you get are way way better than anything that would ever come out of the smartphone. Ever, because matching them with a tiny phone camera requires beating the laws of physics.
This attachment is every blogger, social media addict or web junkie wet dream. Why? Because a smartphone allows you to edit and share your pictures instantly and much much better than any camera would ever do. Want filters? You can use snapseed, Instagram, photoshop whatever you want. You can have a high quality image posted to your blog in literally seconds. Your school just won state championship, boom, you have a nice contrasty low light photo on Facebook, you are at a fashion show and while everyone is sharing grainy silhouettes, you post nice high res pictures, etc, etc... It's a big, big deal. If you think your iPhone camera takes perfectly good pictures, its not for you, understood. People who care about connectivity and good photography should be excited.
While I agree that the pictures of these 'camera attachments' will more than likely be a big improvement over the phone's built-in camera, this is still a (very nicely designed) niche market product. Can't ever see this having mass appeal. Too bulky and cumbersome for the average cell-phone user. And with yearly, or even more frequent, phone model changes, a product prone to premature obsolescence.