Can't think of anything worse than training as a pilot and then my airline forbids me to fly the aircraft.
Do S/O's get a proper Type Rating or are they less qualified than an F/O?
In my Uk airline all pilots recruited are fully trained to operate as co-pilots in a two crew cockpit. Many of the shorter Long Haul trips on 777/787/744/A380 are operated with just two crew.
Extra crew are carried for crew rest requirements on longer trips, but all fully trained on the Type and share the landings as required for recency.
Presumably the S/O scenario is a cost saving exercise by the company.