We often hear sustainability advocates saying that the most sustainable building is the one you don't build, the next best thing being the repurposing of existing ones. This article makes an interesting case of saving unused parking structures and repurposing them for a different program.
It shows how a large dose of creativity and some elbow grease can prevent more the unnecessary demise of buildings and the filling of more landfills.
I also think that from a cultural standpoint, it's important to preserve the best examples of any era's architecture even if demolishing is the expedient thing to do.