I think that too much is dependent on the specific situation. A building should not be disconnected from its history. You have to consider, then, what is it about the building that makes it worth keeping? I may just have a very American view of it, but I think that it is a bad idea to keep every old building unless there is a good reason. They take too much maintenance, and provide much less efficiency than newer ones.
For example, I don’t think an apartment complex should be kept if it is just sitting there. Even if the land is used for another apartment complex, it could be better designed to accomodate more people, more comfortably. Rather than wasting the materials, they should be reused whenever possible. Old wood that can be salvaged could be used for furniture. Old bricks could be used to make a path in the "garden" type area of the building.
However, I don’t think we should demolish or move buildings that are historical or useful…no matter the cost of the land it’s on, a building should stay because it is how we interact with our history. We can put old things away, but old buildings among us are a constant reminder of where we (as a society) and even more so we (as architects) have come from.
I know this point of view is naive and somewhat romantic, and money will always rule the decision-making process, but that’s just how I feel about it.