In this case, I don't think it's so much a matter of dropping loot as a reward for beating the trainer, like how in Pokemon games you receive money when you beat a trainer and lose money if your pets are all KOd. Basically, betting on the results of the match.
I could see getting some kind of reward for beating an npc trainer, but I think they'd only have repeatable rewards (other than, perhaps, a small monetary reward, akin to an easy quest) if there were a consequence for losing, which runs counter to Blizzard's apparently pet battle design philosophy. I think I heard quite a while back that you'd get rewards the first time you beat certain trainers, such as unlocking a new battle pet slot or gaining a new ability, such as an item to use on pets in combat. That may have changed since, though.
As for pets, I really can't see the trainers giving a pet every time they were beaten, or that pet would pretty much end up vendor trash, and I really don't want to have to face yet more rng nightmares. I like the idea of getting some xp or a little bit of gold each time we beat a trainer, but when it comes to rare pets, I like the new wild pet spawn philosophy. I'd rather be rewarded for research/exploration and go catch a pet that only spawns between 5 and 7 pm when it's raining than have to fight one guy fifty times for something that may never drop.