Shohin pots are very small. You need every bit of available space in them to have soil for roots.
“Drainage layers” are needed for larger, deeper pots to promote proper drainage.
As to the quality of your information about bonsai... There is a lot of information passed around that is simply “passed down” and no one thinks anything about it. But, it might not be entirely accurate. For example, wiring. Virtually every book says that when wiring out a branch, the wire should be at a 45 degree angle when viewed from the side. Right? Well, that’s wrong! For maximum strength, holding power, and aesthetics, the wire should be more at 60 degrees! Do they say that? No! They always say 45. Why? Who knows???
Likewise, your “sharp particle” theory is just plain wrong! You don’t want sharp particles in you soil mix. Sharp particles pack. They stack up and become solid. Instead, think of a glass filled with round marbles. There’s lots of air space. Right? Think of a stack of bricks. No (or little) air space.
Akadama, by the way, starts off round. And yes, it does break down over time. But it still breaks down in rounded particles. It works by being open when it’s fresh. Lots of air space that lets roots grow quickly in between the particles. That’s good when you’ve repotted, you get new roots quickly. But as it breaks diwn, the root growth slows down, the spaces between particles become smaller, thus promoting smaller feeder roots, rather than coarse “tap” roots.
Bonsai don’t need “tap” roots because we use wire to tie the tree into the pot. In nature, the tap roots secure the tree to the ground. In bonsai, the wire serves that purpose. So, all that’s needed are the feeder roots. Using akadama helps build a network of feeder roots.
