Here is what Johnston shares regarding the value of good lettering:

If once the principles [scribes] have established could gain currency, what a load of ugliness would be lifted from modern civilization! If once the names of streets and houses... were executed in beautifully designed and well-spaced letters, the eye would become so accustomed to good proportion in these simple and obvious things that it would insist on a similar gratification in more complex and difficult matters.

Compare this with the closest we get to something about the beauty or joy in calligraphy from the now-canonical Mastering Copperplate Calligraphy by Eleanor Winters (which I do otherwise recommend!):

There is tremendous gratification in making an elegant curve, a beautiful letter, or even a good stroke.

I mean, I'm obviously someone who agrees, but the prose here doesn't exactly get me rearing to fiddle around with some nibs and corrosive inks and hunch over tracing paper for an hour. But when Johnston tells me that "much would be gained by substituting, generally, writing for designing", and his editors that "workmanship when separated by too wide a gulf from fresh though--that is, from design--inevitably decays", well, now I'm excited to design some letters.

Beyond that, the wonderful detail given to the other arts of illuminating (burnishing, rubricating, gilding, all of which seem to be neglected now) working towards the "Theory of Illumination", as Johnston puts it, makes me hungry for some rainy afternoons to try them... lowkey think I would have thrived as a pre-printing-press monk