Those are also not nearly as popular, especially these days.
Back in the old days Abyssea - as an example - was extremely congested. It was almost impossible to get certain trials or notorious monsters done.
I like your suggestion about making different requirements - though I would take it a step further and say it might help to randomize the entire system of ambuscade. Have a roulette choose from random battle grounds a group must complete for hallmarks/gallantry. That would add more value and relevance to a lot of content - as well as alleviate the repetition of doing the same fights over and over and over and over... again.