Dropbox has your stuff stored away in a fairly secure facility, while BitTorrent Sync just syncs between machines you've installed it on. That's worth something. Of course, you can just rent a cheap VPS somewhere and have it sync your BTS folders, but I think you'll be hard pressed to find a VPS with 100 GB storage for less than $10/month.
That's easily solved with Backblaze backing up one of the synced machines. Then it's $5/month for unlimited with off-site backup.
I agree with you that this is a very important consideration: Sync services (even Dropbox) are not backup services and shouldn't be treated like they are. You are in an even worse predicament if you are using it to sync everything locally.
Perhaps that actually may be where BT's headed to once they decide to take this out of alpha. Paid storage service for BT Sync that augments sync service.
I particularly like BT Sync that it doesn't require "account" to use.
Also, Dropbox is user friendly. I can recommend it to my family members, and they have a referral program to encourage me to do it. What does BitTorrent Sync do that rsync cannot?
On the other side of the argument, I bet BitTorrent Sync will become as friendly as Dropbox, and perhaps third-party backup services will offer what Dropbox does. At that point, rsync's only advantage would be an open-source license.
It seems to overcome NAT and dynamic addressing very well. All you need is to distribute the small shared secret once (for example, offline via USB, paper, or via the first letter of every headline in consecutive college newspaper publications), and you have the ability to transfer files (and do anything that can be boiled down to that; ex: chat, perform backups, publish videos of cats among your friends, distribute mostly static data between front-facing servers, etc...) without further coordination (for example, constantly renewing a DNS record, overcoming NAT, and keeping SSH or FTP permissions in order). Sure it may not be the best experience for all use-cases from a UX standpoint, but it is very general and, being serverless, it is better from other perspectives. I'm not boxing with you, only responding to your "?" and advocating for more such software.
Maybe use s3fs to store one of your BTS copies on Amazon S3?
That'd cost you $9.50/month just for the storage, bandwidth and transfer costs are a bit hard to predict - storing your mostly-write-only archives are different from your mostly-read-only mp3 collection or your pirate-bay-sourced movie collection collection.
I'm quite excited by this - I've been planning-but-not-doing-anything-about a cluster of inexpensive processors (probably RaspberryPi's or TPLink WR703-Ns) running Tahoe/LAFS. If this busts through NATed internet connections like I suspect it will, I'll probably give up on that plan and just install a bunch of copies of this in various places.
