Ill start this rather general question with a disclaimer... I typically have no idea what I am talking about when it comes to any TMA. I have a rather basic idea of what most martial arts are about, but that's it.
I hear time and again about arts that don't do any sparring in their courses, or curriculum, or whatever you might call it. My question is: why not?
Is there a fundamental reason that a particular art would forbid sparring amongst students? To me, sparring seems like it should be one of the most important steps of training.
Step 1: Learn the technique
Step 2: Practice said technique at low speed against another student or on a practice dummy to get a feel for how it works
Step 3: Practice technique again at full speed in sparring to attempt to simulate the use of this technique in a realistic situation
This is how I was instructed when I dabbled in MMA and when I trained in US Army combatives.
Another question: Why is it frowned upon for a lower ranking student to ask a senior student to spar?
I can't remember exactly where I read this, but I have heard it from someone who has trained in martial arts for a long time. As a lower belt, you just don't invite a black belt to a sparring match. And not because you might get hurt, but because it is just against the rules. Why is that?
____________________________
"A man who has attained mastery of an art reveals it in his every action." - Anonymous
You'll hear mixed views on this. Some say it leads to or teaches bad habits. Some are fine with doing scenario training and training their empty hand techniques in a realistic fashion, in place of sparring. For me, I do both. I train an empty hand SD tech in the fashion that you listed. I also spar. Sometimes we just go right into it, other times we work on a specific tech, and then try to fit it into our sparring. Personally, I enjoy sparring, as it tends to force you to separate the fluff that you often see in the arts, from what's really going to work.
As for your 2nd question..I really have no idea. In some schools, lower belts, until they gain the experience, will always fight a more senior student, not necessarily a BB though.