OK, I looked at some piano roll layouts in DAWs, and found examples with equal spacing between semitones, you're right.

I looked at some music mapped onto this layout. The layout allowed for speedy recognition of lone intervals and simple chords, but no more so than standard notation, and it was far worse than standard notation at showing any higher level musical content (the harmonic function of those intervals and chords in the context of the whole piece or local progression, polyphony and independent voices in general, counterpoint, organization of rhythms and cross rhythms, phrasing, larger compositional structures...). Also, because it's just a homogenous field of repeating piano octaves there is much less to positionally anchor the eye on as it reads through, like a staff with ledger and bar lines and key signature would provide to a fluent sight reader. All this musical meaning is stripped away and to reinstate it would be a case of painstakingly decoding the piano roll rather than just fluently reading the music.

I can see how this layout is really useful in a DAW for MIDI input and editing. As for learning the absolute basics of harmony, yeah it's accessible, if you can't be bothered to learn the basics of standard notation. After which point it's a dead end.

Thanks. The piano roll helped me to look at things from first principles - to see how, say, progressing chords towards a change of key you can pivot into the 'allowed' semitones of the new key. Learning about music, the jargon is quite dense, epsecially with the naming of chords and intervals. Thinking of a fifth as seven semitones (and a 2/3 frequency ratio) and so on made it more digestible for me. But perhaps it is a dead end, I might have to take your word for that. Maybe its something like how in chemistry, learning the Bohr modelof electron shells helps you grasp the structure of the periodic table, but then to really get further you have to forget all that and start again with the valence electron model.