As for why it's interesting and useful theoretically - well, Japanese is of course older than the kana chart, and if you're doing diachronic linguistics I think taking mat- to be the stem is probably a useful frame to think about the origins of the modern conjugation paradigm.
It is equally unnatural to form kana in your head when thinking about how to say something! Or, it’s unnatural to think about whether a verb is godan or ichidan when speaking, it’s unnatural to think about what particles are, etc.
You get an explanation for a concept and you internalize it. The explanation is a framework for you to internalize the concept, and once the concept is internalized, you no longer need it. The explanations are still useful, like romaji is still useful. None of the explanations are necessary, some are useful.
Maybe you get a few different partially wrong explanations of what the “wa” particle means before you internalize it. You’re not getting damaged by those explanations, they’re Wittgenstein’s ladder. (Well, maybe you got some bad explanations, but even the good explanations are wrong.)
You throw a bunch of different explanations at different students, combine it with practice and lots of examples, and eventually it sticks.
> which in this example is not actually even the same consonant
By that standard, the “t” in “butter” is not the same consonant as the “t” in “tap” (try it!)
People are really good at understanding and learning phonology and can really internalize it and understand it on an intuitive level, even if they can’t explain the rules for it. If you’re a native English speaker, you accept that there’s a “t” in “butter” and a “t” in “tap” even though they are not the same sounds (phones). That’s because they are the same phonemes.
Romaji is an effective way of writing down Japanese phonemes.
- “Need to” is a very different criterion from “is useful”, and “visually split” is, well, kind of unexpected to me!
- The comparison to native Japanese speakers seems especially irrelevant here,
- You’re not using your native orthography here, you’re using romaji, I don’t think it’s anyone’s native orthography.
- You probably want to use the Romaji heavily anyway, because that’s how most people will use computers.
When you’re learning a language you naturally multiple approaches to understanding a single topic. You’ll see a word written, you’ll hear it. You’ll hear a sentence and then someone will give you an explanation of the grammar, and you’ll compare that with your notes. It’s multimodal and all of these different modes are useful.
Like how romaji is useful for understanding conjugations. Obviously not necessary, just useful.