Empty rental properties should be taxed at their rental rates, regardless of if there is a tenant in them or not. Wanna rent out slums for 1700 a month, well, you aren't going to pay taxes on that property like the 90k shithole that it is.
To go a little further, if you're listing a house, you should pay taxes on it monthly based on the list price. Seen way too many shithole with a fresh coat of paint hiked 150k above their actual value.
Back when we were house hunting, we experienced several sales where the house was sold for cash at 10-30k over the asking price before offers were even open. If we do sell, I'm sure as hell only selling to a family (not that I'd advertise as much, I'm sure they would and have made fake families to purchase houses.)