Unfortunately it's impractical to keep this information server-side because of memory concerns. We already support a rather large number of users on one not very powerful server with a limited amount of memory. We've had memory problems in the past doing a whole lot less... it would also make it harder to develop fancier client-side features such as user list groups, such as is already implemented for the next SoulseekQT build.
Make backups.. use Windows scheduler to make them hourly, even.
I endorse keeping AS LITTLE user information on the server as possible, for both privacy concerns and to enhance the distributed nature of P2P, so I'm "downvoting" the addition of this feature.
However, the client should have an easy to use "Backup configuration" mechanism that would create a single, compressed file on a user-supplied destination at regular intervals. Maybe even keep the last 3 versions or so, to account for unseen corruption.
In the mean time, if you are not computer illiterate, just setup your own cron/task scheduler job manually, and remember to quit SoulseekQT before backing up the files.