No idea why BitTorrent Sync bothered to release a client that wasn't open source. People who care about their security won't use closed-source programs; and people who don't use Dropbox.
Dropbox costs money, and it uploads everything to Dropbox.
With BTSync, I can sync hundreds of gigabytes over a LAN with very little effort. I do hope a good open source option will be available soon, but for many use cases this is a big upgrade.
Perhaps we could use git-annex for this?