Every switch of workspace forces accounts-daemon to rewrite /var/lib/AccountsService/users/xxx file
I use Xubuntu 14.04 Linux xub-14-04 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux xfce4-panel 4.11.0-0ubuntu1 amd64 panel for Xfce4 desktop environment
This is a Xubuntu-specific bug that happened due to a patch we carry in the distro (which informs accountsservice of the current wallpaper so that the greeter has the correct background image). Also, it has been fixed in Xubuntu meanwhile (https://bugs.launchpad.net/ubuntu/+source/xfdesktop4/+bug/1318307)