Users in various countries are using multiple keyboard layouts. Setting them thru the menu they confuse the System/Administration/Keyboard and System/Settings/Hardware/Keyboard. Setting keyboard layouts in firsboot would help them to use their desired layout right after installation.
Most likely the person who runs firstboot is also the person who installed the system. This might make more sense as an advanced option to gdm say. Of course there is already the GNOME keyboard switching applet.
We are moving more towards configuring these sorts of things in the display manager instead of firstboot.
Adding FutureFeature keyword to RFE's.
keyboard layout selector is present in rawhide gdm.