Often in complex non normals it is very beneficial to have the FO as PF so you can free up your mind and manage the resources at your disposal...but the captain would always be PF for the approach and landing.
And in a complex non-normal in an aircraft such as the A340 there are a number of scenarios ( inertial systems, air data computers, display management computers ) where a captain will not have any computer screens for the approach but the F/O will.
The F/O, if capable, is in a better position to fly the approach and landing than the Captain who would be relaint on standby instrumentation.
Who should fly the approach? If the approach is messed up you are now faced with the relatively difficult missed approach procedure with degraded systems.
Decision?