From urbanist Jane Jacobs in The Death and Life of Great American Cities:

Cities need old buildings so badly it is probably impossible for vigorous streets and districts to grow without them. By old buildings I mean not museum-piece old buildings, not old buildings in an excellent state of rehabilitation – -although these make fine ingredients — but also a good lot of plain, ordinary, low-value old buildings, including some rundown old buildings.  As for really new ideas of any kind—no matter how ultimately profitable or otherwise successful some of them might prove to be—there is no leeway for such chancy trial, error and experimentation in the high-overhead economy of new construction. Old ideas can sometimes use new buildings. New ideas must use old buildings.

Photo is from Treehugger article about an art incubator in Toronto.