"gksu -u other_user .." is broken

I did this test run with Lubuntu 13.10 amd64 and Xubuntu 13.10 amd64 in separate VMs.

Steps to reproduce:
- Install fresh Lubuntu/Xubuntu
- Open the XFCE terminal.
- Type "sudo -i", enter password, and do the following:
# create a secondary useraccount
adduser testuser
# use nano or your favorite editor:
nano /etc/sudoers.d/testuser
     # allow your main useraccount to execute ANY commands as the "testuser" user using sudo. password will NOT be required
     your_main_username ALL=(testuser) NOPASSWD: ALL
     # save the file.
# set permissions of sudoers.d correctly
chmod 440 /etc/sudoers.d/*

- now open a shell as your main user, NOT as root. do:
gksudo -u testuser xfce4-terminal

This should open a terminal logged in as "testuser". But it does nothing.
Add the debug option and pipe to less to view what happens: "gksudo -du testuser xfce4-terminal 2>&1 | less"
Output:
No ask_pass set, using default!
xauth: /tmp/libgksu-wPttP0/.Xauthority
STARTUP_ID: gksudo/xfce4-terminal/2799-0-xubuntu1_TIME2139054
cmd[0]: /usr/bin/sudo
cmd[1]: -H
cmd[2]: -S
cmd[3]: -p
cmd[4]: GNOME_SUDO_PASS
cmd[5]: -u
cmd[6]: testuser
cmd[7]: --
cmd[8]: xfce4-terminal
buffer: -No protocol specified-
buffer: --
buffer: -(xfce4-terminal:2802): Gtk-WARNING **: cannot open display: :0.0-
buffer: --
buffer: --
buffer: --
buffer: --
buffer: --
buffer: --
(a lot more of the previous messages)
brute force GNOME_SUDO_PASS ended...
No password prompt found; we'll assume we don't need a password.
xauth: /tmp/libgksu-wPttP0/.Xauthority
xauth_env: /home/xor/.Xauthority
dir: /tmp/libgksu-wPttP0

The fact that there is output of xfce4-terminal BEFORE gksudo talks about password, .Xauthority, etc. might indicate that it tries to run the child application too early? I haven't read the source code though, please validate this claim by reading it.