That's actually one of my favorite features. That should never happen under the limited circumstances I use it for. If something goes so wrong that my account actually runs out of money before I notice, then I far prefer the default to be "intruder alert, intruder alert, wipe everything". There's a reason it's marketed as backups for the truly paranoid.
Then why bother backing up at all? Buy some gasoline and set your laptop on fire, you'll never get more secure than that.
For me, personally? I do it for kicks. But in general, there do exist many data loads in the world which are valuable to the holder, valuable enough to be worth the low cost of a backup, up until time T in the future. After T, however, they become more of a liability than an asset to hold. A self-destructing backup model is the obvious fit for such situations. Both the positive-sum and the negative-sum periods need to be considered to truly safeguard your data properly.