A non-vacant used property will provide a city with money in the form of sales taxes, jobs, growth, fees and in general prosperity. A town with a ton of small independently owned shops and restaurants (a side effect of forcing property on the market) is more attractive than one with strip malls filled with franchises. It will make people want to live there and drive prices up.
Why should a vacant property pay the same tax? It's taking up the same and paying less, in fact, in many cases, the vacant property is pulling down the value of the city. It's only fair for the vacant property owner to have to pay to blight his community. Such an arrangement is dealing with externalized costs, which is one of the best functions of government.