Comment 6 for bug 48734

Maybe it wasn't clear.
A non expert Ubuntu user has tested the guest mode and find out that his screen is locked afterwards and he could come to the impression that this is safe. Especially since he heart Linux is much more secure than Windows and so on.
So if he has to leave he just switch to guest mode to let a friend, sister, flat share or whoever use his computer in his absence thinking that his personal stuff is safe.
So with this default configuration everything depends on the permission setting of each app. Firefox seems to set restrictive permissions but many other apps not. And even if the settings are safe the standard Ubuntu directories for pictures, documents and so on aren't.

If there really is a need for sharing files between users a standard shared folder would work around the problem.
Btw. every experienced user knows how to change the home directory to make it less restrictive so it wouldn't bother them but it would be more secure for every one.