Don Firth said,"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."
Exactly. A Major chord may always be played in place of a Ma7 or 6 chord or a dominant seventh, ninth...

In the key of C, if we take every other note for 3 consecutive notes and move up the scale, we get: CEG(CMa), DFA(Dm), EGB(Em), FAC(FMa), GBD(GMa), ACE(Am), BDF(Bdim), back to CMa.
These chords work well in most folk tunes and, as Don said, are all that are actually necessary.

If we do the same thing with every other four notes, we get: CEGB(CMa7), DFAC(Dmi7), EGBD(Emi7), FACE(FMa7), GBDF(G7), ACEG(Ami7), BDFA(Bmi7b5 or B half diminished), back to CMa7.

These extended chords are not necessary, but sometimes add colour to songs.

A folk version of Hey Good Lookin' would be played:
|G |   |   |   |C |D7 |G   |   |

A western swing version might be played:
|GMa,GMa7|G6,GMa7|GMa,GMa7|G6,G#dim|Ami7,Ami6|Ami7,D7|
GMa,G#dim|Ami7,D7|