[jellicle]

Is that a trick question? The negative externality is leaving a usable property vacant when public policy encourages use. You're maintaining this situation:

https://squattinglondon.files.wordpress.com/2018/01/ominous-...

and it seems obvious to me why that's bad and public policy should not promote it.

As a public policy, along the lines of taxing vacant properties, we could also simply permit any vacant or unused property to be lived in and used by anyone who came along.