The reason I say it's not useful (as opposed to merely interesting) to a learner is that I think if you're ever explicitly forming a structure like "mat-" in your head while thinking about how to say something or how to interpret something in Japanese, you are doing something quite unnatural, since that form cannot actually be expressed in the language. A fluent or native speaker knows that, eg. to make "matsu" a polite order, the tsu (tu) becomes chi (ti) and then you add nasai; they are not thinking about the isolated 't' consonant (which in this example is not actually even the same consonant - and native speakers know that, considering they distinguish ティ in loanwords!).
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.
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.