The point is that the ET circle of fifths relates to other temperaments, and in fact the only use for a precise construction is if you're going to calculate such comparisons. So it is perfectly reasonable to be interested in both at once. (As I pointed out in my first post in this thread - that boxwood slide rule dated from the 1880s and there are tables equivalent to it in Helmholtz's book, this is not a new idea).

There is no such thing as a "modal tuning". I presume "twelve fifths tuning" is a subliterate description of Pythagorean intonation.

Just intonation has lots of well-established uses in folk idioms worldwide. The commonest musical instrument in the world uses just intervals.