I'm surprised that there are not more people wanting to login to Gnome Flashback and have a working screensaver. You cannot unistall gnome-screensaver because it is a dependency for gnome-session-flashback.
This was the common practice after the initial install of other Ubuntu versions: uninstall gnome-screensaver, install xscreensaver and add it to your startup applications.
This problem was introduced with Saucy and persists in Trusty.
If you want a working screensaver in flashback add your name to this bug.
Here is what you need in Trusty:
1) Add this to a text file:
2) Then save it in your home directory as /home/cavsfan/killscreensaver.sh (of course substituting your username with cavsfan).
3) Enter sudo chmod +x /home/cavsfan/killscreensaver.sh to make it executable.
4) Enter sudo chown root:root /home/cavsfan/killscreensaver.sh to make it owned by root.
5) Add /home/cavsfan/killscreensaver.sh to your startup programs.
6) Add this to another text file:
7) Then save it in your home directory as /home/cavsfan/xscreensaver-start.sh (of course substituting your username with cavsfan).
* this script must not be owned by root or the screensaver will not start.
8) Add /home/cavsfan/xscreensaver-start.sh to your startup programs.
9) Make gnome-screensaver unexecutable with this command sudo chmod -x /usr/bin/gnome-screensaver just to make double sure.
I've tried many combinations and many did not work but the above appears to work OK.
I even entered sudo killall gnome-screensaver in terminal and it killed it but it re-spawned back to life somehow.
It is set to kill gnome-screensaver after 3 minutes because it doesn't start for a while and anything less will not work.
Then it starts xscreensaver after 4 minutes and it cannot be started before gnome-screensaver is killed or it will not start.
I logged out and back in and this worked. Haven't tried rebooting but, that should do.