This demo shows what can be achieved with the correct use of metadata from Flickr images. If nothing else, this answers why you would bother with metadata (as if there needs to be a justification these days). By correctly annotating your content you gain an advantage in leveraging a network of data. Via, Semantically-linked Interactive Imagery: Wow! The Emergent Web in Action and the demo Blaise Aguera y Arcas: Photosynth demo.
Update: What? I've edited this entry. Weird, I'm sure it made sense at the time.