Under Windows, unless I missed something, I disagree with what has been suggested in the above mentioned thread. Databases, catalogs and their backup are… data. Storing data on the system disk is not a good idea, especially when using an SSD drive. So, I consider leaving the choice to the user as a good idea. Under Windows, the %appdata% folder is on the system disk and cannot be moved to another partition. The My Documents folder can.
Actually, I never use the built-in backup feature available in DPL, Lightroom or whatever application using a database. Since databases and catalogs are data, they are part of my general data backup scheme (which is automated and can also be launched manually) as any other data. There’s no reason to handle these data differently. Of course, these files are always stored on a partition (that is, another physical disk) dedicated to data. Backups go to yet another partition (NAS or external drive).