It is NOT a bad idea to switch ON an ADR that has been switched OFF previously by an ECAM line.
It IS a bad idea, though, to switch ADRs OFF or ON without first performing an ADR check procedure as per the QRH, since now we know that two rogue ADRs can vote a good ADR out, typically in icing conditions.
We should always suspect of any ADR FAULT. You can do the ECAM, but if later on you realized that the ADR is good and the others are bad (after the ADR CHECK PROC) then you must switch it back to ON, and the others OFF. As per the procedure.
It is not recommended to troubleshoot ADR issues without doing the ADR CHECK PROC. You might get in very serious trouble.