You could first check via `xev` whether the key presses actually arrive in software. For me they did, but my desktop environment didn't adjust the brightness if an `XF86MonBrightnessUp` event fired.
Manual mapping shouldn't be required for most current desktop environments. GNOME and KDE certainly do, and I think Unity does as well. If your desktop environment doesn't map those keys by default, please file a bug against it.
It's great that Linux makes it possible to manually fix such things, but it's even better when they work out of the box.