I appreciate the input so far. But why do the folks at Hal Leonard, etc. choose to use Minor 7th and not Diminished in their simplified arrangements? Are Minor 7th chords a better "musical approximation" of the full piano score / bass clef?

If you are stuck with an accordion or a chord organ that has dedicated buttons for Diminished chords (but not Minor 7th), are there any rules for chord substitution? Is it me, or did Diminished chords just fall out of favor during the 1960s?