Someone recently posted a question about recovering a drawing after mistakenly deleting the AEC. And sometimes we start editing a file, and forget to do a Save As; so we could open the BAK and recover the latest version of the file.
That makes perfect sense Neil, and I now understand where you are coming from.
... On thinking about it, I am pretty sure that I normally save (manually) immediately before closing DataCAD, so I wouldn't have a BAK of the previous version as I most likely also select to save when prompted. This is not because I think about it, but rather habit from using other applications (e.g. Word) which do not prompt you to save on exit if you have not made any changes since the last save (and that would be my suggestion for DataCAD, but it is pretty much the opposite of what you suggested! )
I expect that you have at least a nightly automated backup, so you could always get the current version from there if the mistake was made the following day, but I understand that it is probably easier just to go to the BAK file (or the mistake may be made before it was backed up). Actually this is precisely the sort of reason why I wrote dhSave, but I'm not aware if anybody is actually using it.