1. Powergaming Tao builds do exactly as I said especially if they start ath the stated level. Non-powergaming.. I assume they can stay Tao.
2. Goku haven't studied any MA, I just recently read the manga. And about others — it's quite possible that they stayed tao which'd explain why they never had flashy effects compared to Goku, Vegetta or Piccolo. (also it can be explained with race or lower level)
1) As I said, I see Technitian being one of two logical advancements of Taos. Either you get the awesome Martial Arts that require level 13+ or you become more of a Technitian. Switching classes as your character changes is less of a way to become stronger, and more of a way to define a character. If you are doing it for mechanical benefit alone then of course your going to become stronger, but only in what you want to do. A pure Tao will be better at MA than a Tao/Technician and with what some of the high level MA do your giving up a LOT. If you don't believe people ever change then I suppose it wouldn't make sense to change classes, but that is why there is the two level wait - you don't change overnight. You choose to change and work on it.
Mechanics vs Story is often a problem, changing your class should be because it is best for the story, the mechanics just support it. But that is a problem between people playing it just as a game and people telling a story.
2) I have to disagree on Goku. It is true he never learned Martial Arts, but it wasn't until fairly late in Dragon Ball that he used any Ki. He was really an untrained fighter, mechanically he would be a Freelancer or maybe Fighter that become a Technician that learned a few MA's as well. It took him till half-way through Dragon Ball to learn any Ki abilities, and by that point he was like level 4 or 5 (able to beat the best people in the world). Further DBZ is not the only example, nor the best. It is merely a well known one.