Emphasis on diatonic.
As someone who knows and understands the major scale and it's modes backward and forward and has known for decades while only having a cursory understanding of all them weird jazz chords, I can say that each day that I study jazz theory it becomes more and more evident that chords is where it's at. One little example in an ocean of oddities, as you must know bassed on your TB profile ... What scale tells you that we commonly raise the 9th on the iii chord of a major scale to avoid the "avoid note" 4th? Ono the other hand, follow the chord and that will save you.
We all learn differently. Scales are very important but put yourself in a less experienced student's shoes. If nobody says "hey, forget the rigidity of a scale for a minute and just trust the chords," only then are they (me) gonna go "holy shtick, now I finally get it." Sure, there are weird and altered exotic scales that were created to facilitate odd stuff (odd stuff like commonly making the V chord major in a minor ii V I, where it SHOULD be minor according to scales most commonly known by regular guys... but the harmonic minor scale was created to facilitate it so that there is an easier, half step between the 7 and 8 ). If one just follows the chords one can get by, especially non-solo instruments.
Again, scales are cool and I'm not picking a fight but I have been there, bewildered by chords that don't fit the rigidity of common scales and the given key signature.... not all too long ago and for a long time. Giving up and just following the chords worked until I have time to more thoroughly understand what is really going on.
So you're… agreeing with me then? It sounds like it.
Because altered and non-diatonic chords are inherently more advanced of a topic than keys and diatonic chords are. Having the foundation of understanding diatonic harmony is important to then build upon and add the more advanced chords.
Sure, there are weird and altered exotic scales that were created to facilitate odd stuff (odd stuff like commonly making the V chord major in a minor ii V I, where it SHOULD be minor according to scales most commonly known by regular guys... but the harmonic minor scale was created to facilitate it so that there is an easier, half step between the 7 and 8 ).
But anyone who has studied how diatonic harmony works would not think this is "odd" at all because they would know that in order for a functional V-I relationship to exist within a minor key the 7th scale degree must be raised to make the V chord major, thus the origin of the harmonic minor scale. In other words, a major V chord IS diatonic in a minor key.
So having a full grasp of how keys work makes the example you gave a non-issue.