I've only flown various Boeings, the latest being B738NG. LNAV/VNAV & RNAV approaches were authorised, BUT any ground based aids that helped confirm the FMS guided approach had to be tuned and monitored as back-up. Why would Airbus not tune and monitor a back-up aid, whether it be manually tuned or automatic? Is not a common standard in aviation to have back-up systems for most critical areas. If there are 2 fuel pumps, 2 hydraulic pumps, 2 braking systems, 2 gear & flap systems, 2 pilots......then why not have 2 independent NAV systems in use if they are available? Why rely on only 1 to bring you in close proximity with a concrete grave?