The problem of a one size fits all students, is that gifted and talented individuals who truely learn the curriculum faster suffer emotionally and can walk away because of a ranking system that doesn't flex.
Keeping the gifted challenged is an instructor's silent nightmare.
My instructor said something in one of my classes that stuck in my head, "There are two times things are really enjoyable, when something is new and when you gain a degree of mastery. The trick is to get through the period in the middle."
Depending on how you view martial arts (are they just a physical activity or something more), I would say that period in the middle is the most important. It's the time when you say, "Maybe I'll skip going to class today." And you have to decide if you'll listen to that voice or go to class anyway. So I would say making students wait a certain amount of time before testing or learning new material is actually a way to teach students to grow. They have to learn to self-motivate and get through the boring bits. The instructor can guide them, but it's really up to them to do it for themselves.
So yes some students will walk away if they are made to wait X amount of time before testing, but the ones that stick it out and push through the middle part will be rewarded with not only advancement in martial arts but also in personal emotional growth. YMMV but I think it's usually the "gifted" students that often times need to learn how to self- motivate the most (because things come easy to them) and probably most benefit from being made to wait. So I don't really see any problem with a hard rule about how much time there must be between testing, especially between higher ranks.