I use my Network Railcard about a dozen times a year at most, so don't see any point having it in my wallet all the time (especially as they never gave me a plastic holder for it for the last 2 years and the old one has disintegrated...) . Mine goes in my wallet, but if my girlfriend is using hers, she just chucks it into the handbag as the purse is full..
I suppose carrying something as heavy as a normal rail ticket is a major challenge for your wallet.
Oh well, if you forget it when travelling, your wallet will have even less to hold if you are caught travelling without it.