Correct, you will get all your data off GPS when no sensors paired. Its fine, just not as super accurate as with sensors but really very little noticeable difference I am sure.

What Steve said, you can make different activity profiles and name them whatever. I created a profile for all 4 of my bikes, then I customize the data screens to each bike profile.
I only have a power meter on my "best" road bike so I only have that turned on in a data filed for my one bike.

in short, you don't need the sensors to use the 520..heck you never need a sensor for basic data.