THIS IS A REALLY, REALLY BAD IDEA, DON'T DO IT. IT WILL ALMOST CERTAINLY LEAD TO A CORRUPTED BACKUP OF YOUR FILEVAULT SPARSEBUNDLE.
When a disk image is mounted read/write, Mac OS X can and will rearrange files on it whenever it needs to. This often happens when an application requests space to write to a file. Such rearrangements will lead to writes to both the file space itself and the disk catalog. Since stuff is always being written to ~/Library/Caches and ~/Library/Preferences, changes are always occurring to the sparsebundle when it is mounted as a FileVault home directory.
The author only tried mounting the image. He did not run hdiutil verify username.sparesebundle on it to check that the backup of the sparsebundle was truly intact. Even if it was intact once, there is no guarantee and in fact a strong likelihood that subsequent backups will be corrupted. There is a reason why Apple does not do portable home directory syncs of the sparsebundle while the FileVault sparsebundle home directory is in use. Setting up a sync to the file server using Sync Services would have been trivial, but the disk image corruption and sparsebundle band locking issues would have been much too difficult.