It not that easy. Last one I went on was several years ago. Return from Gatwick to Glasgow - so no jolly by any imagination, just wanted to maximise the time on my own sectors so questions were understood both ways.
Got up early on my day off, to travel eventually at NATS expense what a hassle that was to get authorised. It was an official trip arranged with the airline.
Got to the gate, they new nothing about me, crew were waiting in ops but I had been told nothing about this. Despite several requests at the gate and eventually through the despatcher I was told the jump seat was most unlikely!! Nearly went home at that point, persevered - got on the plane and met the crew who obvioulsy knew the score. Long day, hour plus drive home then the expense claim to submit and ensueing arguement about why they should pay after they had agreed to do so.
Might explain the lack of uptake.
Personally I think every two years or so it should be rostered, with expenses on one of our extra non operatinal duties - management normally end up writing these off due to one fook up or another, so at least get them booked and used for the value and benefit of all concerned