the thing is, the .bak file is only the current drawing prior to the last save. In your case it would have been a massive help but those cases are very rare. People on here keep asking how to revert to yesterday's revision. If you opened the file today, QSAVEd after 30 minutes, did a bit more work then saved on exit yesterday's revision has now gone forever - unless you did a daily backup.
I fail to understand how companies can neglect this step. Servers crash, files get deleted. All yesterday's work was done to a wrong markup. Being able to recover an old file must surley make good buisness sense!