Broader privacy implications aside, it's actually kind of neat to be wearing a device which can identify when you're in a particular space and how long for. We have a volunteer tech group working on projects at our local museum and one of the guys implemented a fitbit scanner to identify when people were present and how long for (which is useful, as bureaucracy dictates we sign in/out for fire and visitor-tracking reasons). Every few minutes it broadcasts a request for fitbits, and all those within range respond. They return a mac which can be linked back to a fitbit account, if the user has authorised us to access it, which makes it a bit easier to identify the person who owns the fitbit. We could probably replace it with another sign in system, but passive is kind of neat when you want it.
I assume resolving the identifying problem wouldn't be as easy as using a random mac?