I'm with Pappy on this. As I understand it in the past when we first got wx radar it used lots of power and several a/c in the circuit at the same time meant LOTS of rf which could interfere with the old ATC radars (Smithy can you confirm/deny?)
When I ask other pilots why the switch it off none has ever been able to answer.
Generally its a very badly taught subject in ground school (if its taught at all!)
its free! do you switch off the ADF just because you aren't using it?