It seems like this idea would be harder than we can first imagine.
I just recently made a decent synth of a place, but decided that I could do better. I went back on a different day and started taking more photos. Although I though that there wasn't any real difference, even just the difference in lighting was so huge that photosynth couldn't recognize it as the same object/place, even though nothing but the lighting changed from sunny to overcast.
Time and light conditions seems to be a big part of the synthing process.
Changes that hardly register with us stand in the way of synthing.
It would also seem that if we wanted a quick fix to match up different people's synths, we would have to be able to force one on the other, or, somehow have it compare the resulting point clouds? Of course, at this point, I don't really understand the value or possibility of what I'm suggesting.