Love the idea of your plugin but my DBs on all WP sites get backed up flawlessly and emailed to me already. What is *really* useful is being able to backup files instead of my old way: rsync to Amazon S3.
Since two of my sites have podcast directories within the wp-content directory, the backup is HUGE (one is over 2.5GBs and the other is 1.5GBs). There is no reason to have 15 separate backups of all my files. In fact, what I would like to have is a checkbox for each directory and subdirectory.
When I instruct clients how to manually backup files (since we set the DB to backup automatically) I have them backup .htaccess/wp-content/wp-config.php and any other directories they've created in the root. Anything else isn't needed for a restore.