In which case you most probably need your sby horizon ??? To continue your landing in cat II/III conditions Or to go around in case of PFD / IR / elec failure (leading to a PDF failure) ???
certainly not the 1st one .... Maybe it's legally required to have an operative sby horizon to commence a CAT II/III APP ... But does it make sense to stop an approach of an autoland capable airplane In order to start a go around maneuver as long as your PFDs are operative and knowing that your sby horizon will be u/s during this maneuver ???