Assuming this is a zone where same rider can cross enter AO and AA, the only real advantage I see for the trainer/spouse situation is that the rules require that the rider also own the AA horse. And ownership includes owned by a spouse or immediate family or a company that the rider owns. So, the trainer barn situation could have a string of sale horses in its name that give the rider perhaps a greater pick of horses than the average amateur. Although of course wealthy amateurs are out there with multiple horses, some of which can’t/no longer do the AO height.
In zones where cross entry is not permitted, at least back when I was last showing in these divisions, the work around would often be to find a very good children’s catch rider to take the AA horse around.
Growing up at local shows and C shows, there often weren’t enough people at the higher heights to have many divisions/ages run separately. And, sometimes even in the juniors, the Jr/AO were often combined. And before you say that’s unfair to the AO riders, they tended to have more $$$ for fancier horses. I think it was good competition and taught one how not to be a sore sport.
I did take my current horse to a schooling show last year pre-COVID that only had open divisions. I entered the 2’-2’3" division just for mileage on my new to me and new to hunters import. I thought it would look bad if I entered the equitation against novices and kids, but since my horse wasn’t used to the back to back trip thing, even though we put together a winning first hunter round (winning at this level, not like we scored a 90), the second round consisted of jumping the first jump, horse started screaming at the far end of the ring and bucking. So, we entered the eq. division rather than wait for the next height in the hunters. Including the flat, since it was his first flat class ever and we spent a lot of time pretending not to spook at the broken down outside line stacked against the fence. A couple decades ago, I was in the top 10 in the flat of the USET Talent Search finals. And I did win this flat class against some children, and I was moderately embarrassed by that but also glad that horse only slightly jumped at the loudspeaker, had gotten over melting down at applause by this class, and only sort of passaged in the sitting trot. But instead of anyone showing any sour grapes, a lot of people were super nice and complimentary to me and my horse, and I think I made the right choice of division for him at that time.