The web site http://www.tonalcentre.org states that there are only two "tonally effective" modes of the diatonic scale (viz., the Ionian and Aeolian modes) [http://www.tonalcentre.org/Scales.html] and that there is only one "tonally effective" mode of the harmonic minor scale (viz., its first mode, i.e., the harmonic minor scale itself) [http://www.tonalcentre.org/Harmonicmi.html]. It indicates that, to be tonally effective, a mode must: "support a tonic triad. That is, it must have a chord which serves as a chord of rest and completion, as the tonal centre against which all the other triads are measured and towards which all gravitate."
I find this definition to be ambiguous, and I have not found any other definitions. I wonder, for example, why D Dorian mode does not "support" Dmin as its tonic triad?
Could somebody please clarify the definition of "tonally effective"? Thanks.