Curious to know how many people consider the bonsai process as a means to a final design that is then maintained from then on, or if people regularly restyle and rethink their trees. I ask because a lot of what I've been reading online and in books seems to lean towards designing a tree, growing the tree into that design, then maintaining that design. I've seen many great photos of bonsai that look like "bonsai" and not enough like the full sized tree in miniature.
Getting back to your original question, here are a few more thoughts to consider.
Bonsai, is a work of art, that is (theoretically) meant to last for generations. Its life span far exceeds human life. So, one has to assume, that more than one person will work on it. This is why following a structured approach makes sense. Just imagine what would happen, if every apprentice and student would try to impose his/her individual ideas, whenever the tree was worked on. The bonsai would become a mess, and no particular design would take hold.
Having said that, there are times when a major overhaul in the design is warranted, when the tree was neglected for a long time.
One also has to consider the importance of proportions between the size of the trunk and branches. A pine tree, and many other conifers do not back-bud from old wood (same with certain broad-leaved species). The implication of this fact is, that if these proportions are not carefully preserved by a systematic maintenance schedule, the tree can be ruined forever, with no chance of restoring the original balance.
These are just a few reasons why a systematic approach to bonsai maintenace is inevitable, and as a result, the chaotic and spontaneous structures that one can see in nature are difficult to achieve, over the long period of the bonsai's life.
It is very easy to be spontaneous and casual, when we are in the process of developing the initial design of a new tree. But once the final design is achieved, and the design needs to be maintained, that is a different ball-game alltogether. One also needs to consider the size of a bonsai collection. If you have 50 old trees in your collection (or it can be much larger), spontaneously changing the design all the time would be highly impractical. However, it is a good idea to periodically re-evaluate the design of each tree, and make some improvements on them. Over time, trees inevitably change, and they can out-grow their current structure.
We tend to criticise the systematic approach that the Japanese developed towards bonsai. But this approach made it possible to maintain a bonsai for hundreds of years, through many generations. It is the only way that this can be done, on such a large scale.
When you have a complex task to perform, you can do it spontaneously, or you can have a plan. If you have to do it a thousand times, you better have a plan.