Ok, I don't know why you, of all people, don't consider security to be a valid reason for using tarsnap; but if you don't like that, how about

* Tarsnap's snapshotting model of backups is very efficient (equal to or better than incremental backups) while providing the flexibility of having archives behave as if they're completely independent of each other.

* Tarsnap is priced based on actual usage, so if you don't have much data to store, it can be incredibly cheap.

* Tarsnap is written by someone with the expertise required, and who is willing to take the time necessary, to get things right.

That last, of course, is relevant to security; but it's also relevant to simple robustness, too.

I don't have much hope that anyone will automatically see anything. This is why I go out of my way to educate -- explaining things like why simple code is less likely to have security flaws, pointing out my expertise and experience, and demonstrating that I'm not afraid to have people look at the tarsnap source code.

There's a reason you designed tarsnap the way you did. It's not a little bit different from Mozy. It's a totally different model. Explain how in simple terms. And remember even if your beachhead market is sysadmins, most sysadmins don't code. Don't talk down to them, but don't make your value proposition be about code.