That’s not good use of space, storage is still expensive where I live (partly because of piracy tax). A simple copy to “Windows.old” will not save the bootloader so it won’t really work as a backup – it’s dead space now. Your OS partition (and the EFI one) must be copied bitwise if you need to back them up (but as you correctly mentioned, you probably don’t).
It makes a lot of sense to separate installation and data, especially when the PC will get used by old people (who are more likely to screw up something badly) or power users. And making backups of data is as easy as robocopy D: F:Backup /mir. Yes, I also have lots of experience using and managing Windows PCs in my family and work.