Some folky songs that would flop without the Ma7 chord:
-David Bradstreet's Rennaisance
-Fred Eaglesmith's High Heels In The Rain
-Mark Rust's Class Reunion
-Mike Smith's Hobo's Mandolin
-Jerry Jeff Walker's Mr. Bojangles
-Mari's Wedding P.D.
These tunes all use the C, C/b, Am, C/g progression. The second chord, C/b has the notes C,E,G and B, making it a CMa7 chord.
...or what about Jingle Bells (C)Jingle(CMa7)Bells(Am)Jingle(C/g)Bells

I'm not crazy about using a Ma7 chord in a song like Skip To My Lou, but I do use them in Hey Good Lookin' and Jack Of Diamonds for instance.