I use major 7ths often, but mostly for jazz or pop (e.g., "The Christmas Song" or the final chord in "Wonderful World"), show tunes (e.g., "My Favorite Things" -- the second line of which really needs the maj7), contemporary folk songs (e.g., Bill Staines's "River"-where going to Dmaj7 from Dmaj on the second syllable of "river" at the beginning of the chorus really makes the harmony work).   Never tried them in bluegrass or classic country or Child ballads (though some of the suggestions above sound intriguing), but I did work out what I jokingly called an "Irish jazz" arrangement of Wild Mountain Thyme that used both a minor 7th and a major 7th and sounded good to me and was appreciated by my audiences too.

Anyhoo, there are a lot more than 2% of the songs in my repertoire that call for or at least work well with at least one major 7th and or 9th or minor 6th.    I don't throw them in just to be cute or artsy-fartsy. I just like the 'textures' that chords like that, plus the "demented" chords, can add to a lot of songs.    OTOH, there are some songs that just plain don't need more than your basic 1-4-5 or maybe even just 1-5.   I've seen piano arrangements of "O Tannenbaum" that call for at least 7 or 8 chords, but unless you're throwing in some funky harmony parts, I cannot see (hear) how that song calls for anything but 1-4-5.    (shrug)