The FDs are no use if you have to get the PNF to continuously programme them to show a sensible command. Hence Airbus recommends that they be switched off. Basic rule - leave them on if they are giving useful information but OFF if not. That's why they are switched off when you go visual on a non-precision approach but stay on in the visual portion of an ILS approach. On the ILS they are following an external reference which is still useful guidance.
With Dual Rad Alt failure you lose the autopilot due to loss of gain programming which reduces the autopilot sensitivity as you go down the "cone" towards the runway.
This failure is not well warned in FCOMs and can be a real pain if you are not aware of the implications - no AP, FD, APP mode and DIR Law, Conf 3 appr, +10 kts, +20% on landing distance. The big problem is that the ECAM only tells you about most of these when you put the gear down and you go into direct law. You need to know that when the ECAM tells you that you'll be in DIR LAW with gear down, you should look up FCOM 3 to prepare for the implications.