I suppose it's pretty impressive to be able to do that, but apart from novelty value etc I don't see much of a real application for it.
When I take pics of places, I do it cos I have seen something I like the look of and I have composed a shot and want it to capture a memory etc. I can't see myself wanting to then add that picture to a huge collection that can then be built up to form a 3D image and viewed by the world over. And also I won't be particularly interested in seeing other peoples images of the same place.
Also, will there be measures to prevent rubbish quality or badly taken pictures ending up being used cos that could make things look rubbish etc. #
Valve
Maybe not applicable for all, but if you took a crappy phone pic of Trafalger Square for instance, got home, and there were people in the way, or you didn't quite get a building in, Photosynth will take your picture and montage all of the other pics, so you can pan around and see all of the square with higher quality pictures.