I find that folks with experience outside their primary art are better questioners. They are more likely to explore the movements and focus on what works, and tend to develop a deeper understanding of he principles. They are - to use
@drop bear 's term - less dogmatic. I believe those are desirable qualities in an advanced student.
OK. I just never encountered that in the schools I attended.
As a teacher, I personally thought if a person didn't have knowledge from other MA experience, it was my job to ensure they did completely understand what I taught. I had a good idea what was going to be harder for them to understand because of the things I didn't always understand when I began to study. Other things I could tell by watching what they did and if it was successful or not, and if not, why.
For that reason I didn't care if they had previous training or not, unless it interfered with what I taught to the detriment of the technique. I began the study of Hapkido expecting the rigidness of my previous Tae Kwon Do training. There, everybody did everything the same way. Period.
To my consternation, I was told the Hapkido I was being taught had no rules on how to do a technique. The way taught, having been perfected over a long time was expected to be the best way. But what really mattered was whether I could achieve the correct result of the technique efficiently. And truly, usually the way taught was the best way.
I wasn't encouraged, but I had the liberty to try another way, whether due to my lack of understanding what was taught, or my body or whatever. On the few occasions I did so, I always gave up on my inefficient experiments, and came back to learn the way I had been taught.
As a teacher, I was always willing to answer questions, but frankly, I didn't get that many. I was teaching proper ways to do things. Sometimes beginning students with experience in Chair-Kung-Fu might ask the plausibility of some movie or TV scene. But not more.
I just can't quite see your thought process that you think a student has to bring experience and knowledge you aren't willing to teach if they didn't have it when they arrived. And, they might not be promoted to BB because of that lack.
I don't know. Maybe I'm too dense and I still don't understand your explanations.