Hi,
cant comment on the Airbus as never flown any of the types.
Regarding the augmenting crew, i think they are a valuable asset to be used, particularly for the walkround if its raining !.
It does not make sense to prevent an augmenting captain from doing a walkround, when in the FTL section of the FOM is says we can have a dispatch crew perform all pre flight duties. After that is done the operating crew board the aircraft and do the flight.