Don Firth [[ ... a major 7th chord ... is a "color chord," and like ALL color chords, they should be used with judgement and discretion. If at all.

As to the use of major 7th chords (or 6ths, 9ths, and other notes added to the basic triad), these are "color chords" and are never actually necessary, beyond stylistic conventions.]]

PHJim: [[Exactly. A Major chord may always be played in place of a Ma7 or 6 chord or a dominant seventh, ninth.]]

I guess it all comes down to what one considers "necessary." Where would graphic art be if painters used only primary colors?   "Coloration" is a key ingredient in painting, music, dance, etc.   Some styles of music (e.g., bluegrass) not only don't "need" chord coloration but maybe even lose their identity when chords/harmonies with too much subtle "coloration" are added, but others, including some folk, can be enhanced greatly by not sticking to just "primary colors."

Close harmonies, distinctive harmonies, etc., on the part of vocalists tend to call for things like maj7ths, 9ths, diminished, or augmented chords. And these can be what makes an arrangement of a song really striking and distinctive.

I'm certainly not calling for "color chords" to be tossed about randomly -- I gave the example earlier of "O Tannenbaum," which I still don't think benefits from more than the 1-4-5 set -- but I would definitely not say that such chords should be used with "discretion - IF AT ALL."   That is like telling a landscape or portrait painter to use subtle shadings and color mixtures sparingly - if at all.