I do use Plesk on my server to back up system files like email and control panel files for the system. But my web site has well over 20,000 files and is too large to back up. I do maintain a source copy of all the pages on my local system. Those copies do get backed up to 2 places (on site and off site) each night. Additionally, there is a real time backup of changes using Carbonite. It was only the ubbthreads directory that isn't in my souce copy locally since it dynamically changes. I wrote a script to back that up nightly.
So now, I think at most my exposure is one day of new/changed posts and subscriber activity. I can live with that.