One should also consider a logarithmic decrease in backups: while I'm writing, a backup of the state a few seconds ago is useful, but I don't need every second from weeks ago. Say the system keeps a backup of every few seconds for the last few minutes, every few minutes for the last few hours, every few hours for the last few days, every few days for the last few weeks, every few weeks for the last few months, every few months for the last few years--the odds are then pretty good that I'd be able to revert to a version I find useful, but not requiring an awful lot of versions.

Back of the envelope: 5 versions/minute for 5 minutes = 25 versions; 4 versions/hour for 8 hours = 24 versions; 3 versions/day for seven days = 21 versions; 4 versions/week for four weeks = 16 versions; 1 version/month for 4 years = 48 versions, so a grand total of 134 previous versions of a document, which really isn't that much space, particularly assuming efficient differencing algorithms.

Then there's software that's designed to allow lay-people to get stuff done with the promise that it will hide the nasty things of the world from them.

Microsoft Word really is the perfect example of this. The main use-case it's built around is being a lovely place for a new user to write and print a small document. It captures users from this simple use-case, and then tries to upsell them to large document and multi-user scenarios with madness like mail-merge wizards and sharepoint.

The promise of Microsoft Word is to bring functionality to users while shielding them from the nasty realities of the world. It's reasonable to be vicious about it when it fails to protect the user from themselves.