Time Machine could easily be replaced by cron and tar or some other local backup option. Other backup providers are easily found.
This is not to say anything about git-annex's usefulness or the value of supporting the developer. It just surprises me when engineers are still struggling with backups. I can kind of understand "regular folk", but that's not really the git-annex market, either.
We need integrity checking both at file-system level and at backup-solution level. If I'm not mistaken, with Time Machine on an HFS+ volume, we don't have the former, and the latter is file-based and not block-based (making it not very efficient, space-wise).
I haven't used it yet, but the git-annex assistant is trying to target that niche, with a goal of an easy-as-Dropbox user experience that hides the technical aspects of git-annex under the hood: http://git-annex.branchable.com/assistant/
It's not for snapshotting or mirroring. I used to use CarbonCopyCloner to clone my boot drive to an external USB drive every morning at ~4am, so if the internal drive on my iMac died I had an immediate bootable replacement.
(And when the internal drive DID die, I just rebooted the machine and took nearly a year to get around to replacing the internal drive!)