In an effort to return to the original thread :
When I was a spotty sprog, I always got the Skipper to sign my logbook in the remarks column for that flight and enter his licence number. Proof for the CAA and any future employer, if required.
I do not remember any F/O asking me to do this for the last 6 or 7 years. If this was a requirement it would help to stamp out these individuals for whom there is no place in this profession.