The other thing is, as with almost all web-based solutions, you're being tracked by various scripts in the background (Google API/JS, etc.). So, if you're paranoid, you're better off with a client solution.
Developers can extend this platform with as many plugins as they wish - as an open-source solution, it's totally open. (I believe fragmentation, more specifically increasing the number of less advanced solutions doesn't help...)
Being a live webpage also give us, as the developers, much less headaches in terms of protocol compatibility -- We maintain just one "version" of Sharefest client at the same time.