Your idea about computing the photosynths on Photosynth's computers is not necessarily a bad one. To hear some of the original Photosynth team explain why they didn't set things up that way over three years ago, watch this video:
You will hear, in that video, that doing the 3D reconstruction part of Photosynth is quite computationally taxing and for that reason they chose to do it on users' own machines.
In the video, Blaise points out that one of the places that cloud-computed synths might make sense in the future is for mobile synthing. You can see that even in 2011, the Photosynth mobile app for iPhone still does not create photosynths, for lack of power on phones. Instead, it only creates panoramas.
In previews of a future feature called 'spin movies', cloud computing is always implied: http://bit.ly/spinmovies