I think part of the issue with the Bubishi is the lack of context (not really any extensive readable explanations of diagrams) and some dubious looking techniques. Some of the stuff in the book is clearly effective like the scissor leg takedown. So it's interesting seeing clearly visible techniques that are outside of basic techniques (advanced & not really that common) turning up in a very ancient martial arts text.
The bigger issue is that the book is littered with effective and dubious techniques which makes me question who actually authored it. There doesn't really seem to be any real mention of the bubishi as an important martial arts manual as well in China - as far as I know. It wasn't required reading which makes me think it might have been some random ancient text written in relative obscurity.
Was it really that fundamental to Karate is what I suppose I'm trying to get at? I know Chojun Miyagi was very fond of the text. But I'm not sure that it really was that fundamental to karate. I think it's something that some instructors carried around and maybe drew some inspiration from - like we might draw inspiration from something we read on the internet or a book we come across.
I feel like context matters here - the Bubishi might have been very useful in an era where you had to physically acquire books to get hold of information or rely on 1-2-1 communication. I think that might be the only reason why the Bubishi is still talked about in Karate circles because it was one of the very few martial arts manuals that instructors were able to get a hold of that had some relationship to Karate.
I agree I don't find it nowhere as useful as some of the other books I have. It would be towards the bottom of my list if I'm honest.
I know that Patrick McCarthy's group spar MMA style (it's great that they do) but I'm not so sure that the same applies with Iain Abernathy's group. I haven't really seen any of his guys spar with kata applications - at least from what is available online. All I've really seen is drills - which are great but no real replacement for live sparring where anything goes.
It's definitely an interesting blend of effective and esoteric, which makes it hard for less-discerning martial artists to work out what to do with it, I suspect. I can see that causing some people to think that, because some of the techniques are ones they know work, the others must work too, and they "just don't understand them yet." I would tend to agree with you that it was not nearly as pivotal a work to the development of karate as it was made out to be. There is a notable book from 17th Century China, called Wu Pei Zhi (or Wubei Chi, depending on dialect), which is often cited as being the point of origin for the Bubishi (being another possible pronunciation).
The trouble is, it isn't a single book--it's 240 of them--and, of course, it's also entirely in Chinese, which puts up a language barrier to determining how true that is. I will say, however, that I once perused 900 pages of it, and nowhere did I see illustrations like those seen in the Bubishi. At the time, I was rather surprised, but in hindsight it makes sense. The Wu Pei Zhi was written by a naval officer in the Chinese military, and as such the majority of illustrations have to do with large scale maps, battle plans, troop formations, etc. The closest thing to the Bubishi illustrations that I could find were a mere handful of pictures that show how an individual should hold a spear. Of course, there are over 10,000 pages, total, so I only got to see a fraction. I think it would be very interesting to see the rest of the illustrations, and get some translations for the accompanying text, if it exists, at all. In the end, I find it to be more of a historical curiosity, rather than a real reference book.
As for the way Iain's folks train, I've seen some personal video of it, and talked with him about it, before. Like me, they do a variety of different sparring methods, including MMA-style sparring, kickboxing-style sparring, grappling-specific sparring, and scenario-based sparring. That spectrum lets them cover a wide array of skills, and incorporate both sporting methods and self defense methods. Of course, the drills are what people really look for on the internet, and want him to come teach, so that's what you'll find online. I will also say that while he definitely promotes pressure testing and sparring of various types in his seminars, there is no way to say how many attendees actually do that part when they get back to their dojo. I have known plenty of people to attend seminars, memorize a collection techniques and drills, and call it good.