Is loading your desktop before login safe?
As @eyemyth said above, the script above is the equivalent of choosing "Login Window..." from menu bar when you have user switching enabled.

The only security 'issue' I can see with doing this is that momentarily your desktop will be shown until the computer calls the script - depending on how many scripts you have or how busy your login is that may not be instantly.

Also, someone (correct me if I'm wrong) could boot into single user mode, delete the plist file and then reboot - though this is an extreme.