WmNussbaum, you are right that not every building is going to find a reuse, and that sometimes buildings are demolished to put in something new. But you ignore the fact that many, many buildings can be and are reused, and that many buildings have been demolished for no gain whatsoever.
One way to look at it is that empty buildings have far more options than empty lots: they can be refurbished, adapted to a new use, or torn down and replaced by something new. On the other hand, empty lots require entirely new construction. Given this, how exactly are empty lots more beneficial or responsible?