I have made very, very little use of airline staff travel when I have had it available to me for the simple reason that it is so very unreliable. Airlines are doing everything that they can to fill seats with confirmed bookings so the chances of getting a 'spare seat' are very rare. Having spent 11 hrs on a cabin-crew jump-seat and many nervous hours wondering if I would get a seat, for a price that is the same as, if not more than, the cost of a confirmed seat booked 'on-line', that cheap confirmed seat is "worth" far, far more than any ID90 ticket.
The airline world has changed. If you plan carefully, cheap seats are available. ID90s are now just far too unreliable to have any real value.