The A320 34-10-05B IR2 MEL OPS procedure additionally calls for Air Data switching to FO3. When ADR 2 is working fine why replace its data on the FO PFD when only IR data is lost?
Even if ADR and IR perform two independent and separate functions, they are the same hardware object (example). A IR2 malfunction is a malfunction: you do not know, in general, what is causing the problem of IR2 and it can possibly be a common problem (maybe undetected on ground by the ADR2).
For example, in the ADIRS Honeywell manual you can find in the IR FAULT chapter:
The ADR function of the affected ADIRU may possibly still work, even if the IR function is not working. Cross comparing air data information gives the pilot some idea of the validity of the ADR data for that ADIRU.
So you have no guarantee that if IR2 is faulty, then ADR2 works. This is why, for precaution, you switch F/O on ADR3.