It is probable the requirement to have the "Subjects" goes back in history when the "Subjects" were for the Senior Commercial Pilots Licence of the 1960's. In those days you could not hold an ATPL unless you were employed in an airline.
In Ansett Airways (for example) promotion to a command was based mainly on seniority and as it was an industrial requirement that you had to be in the pilots union to be an airline pilot, union seniority helped (IIRC)
Like now, the "Subjects" were a pain in the arse but to hold a command you needed to have them (like now) under your belt. There were a few first officers who held Second Class ATPL but who either couldn't pass the "Subjects" or didn't want the responsibilities associated with a command. Because of their seniority in the company (some had over 15 years seniority) their pay was very generous and they could bid to select what flights they wanted. Some didn't want to work that much (fly) and bid reserve.
Eventually their seniority number came up and with it came command training. But because they didn't have "the Subjects" they could not be given the required training. So with all those first officers hanging around on top dollar and protected by the seniority system and bidding not to fly unless it suited them (a hot blonde waiting in Sydney for them during an overnight) the airline finally took the hard line of get your "Subjects" passed in the next 12 months or lose your job.
It is possible current operators use the passing of the ATPL theory exams as proof you have met a certain level of professional knowledge regardless of hours flown.