The mutability comes into play when you want to serve the content. Youtube/Flickr pretty much operate on full replacement, when you're done fiddling with your f

The mutability comes into play when you want to serve the content. Youtube/Flickr pretty much operate on full replacement, when you're done fiddling with your files you publish them and they never change. This makes it easier to serve the content cheaper through caching and CDNs. That partial sync is pretty much Dropbox's technical advantage right now. Change a bit and it tries only sync a bit. Now imagine how difficult that would be to serve with multiple layers of caching.