FO's do do announcements - I think what speedrestriction was asking (with regard to the humour being left seat only) is that there will be a fair number of FO's who respect their position in the food chain, and may not want to attempt to lower the perceived professionalism of the flight deck without an approval from the Commander.
I'm not suggesting that they SHOULD - I'm suggesting that their will be those whose personal attitude to the position they hold may place their own limitations upon themselves.