Many have tried to sing with the Muppets. It's tougher than it sounds.
No. 1, no matter who you are, you won't be as charismatic as your co-star. Recall the tendency of reporters interviewing Henson and Kermit to put the mic in front of Kermit (Carson did it a couple of times). That, boys and girls, is star power.
Number two, it takes a certain kind of voice to make it work. Generations of pop stars have tried hanging out on Sesame Street. It's an unforgiving place. Oh, everyone does OK, but it's rarely great. Sometimes it destroys the song entirely. For example, “I Don't Want to Live on the Moon,” a Jeff Moss song sung originally by Ernie, is a complete showstopper, a simple ballad about traveling and missing home that will reduce grown-ass adults to tears. Amazing song in its original form. But add Aaron %$#@ing Neville in a duet and you have little but oversung mush. Bleh.
But John Denver…well, John Denver was something else. When people called him “the human Muppet,” they were usually making fun of him. But Denver's let's-call-it-mellow vibe and patina of fundamental decency made him an ideal addition to Hensonworld, not to mention his telegenic look. (Think about how many really big stars you've seen on TV and how few of them you can describe in detail. Denver's look is etched in everyone's memory.) And his voice never overwhelmed the songs
All of which is why A Christmas Together holds up as well as it does. Denver and the Muppets crank through the “Twelve Days” (only slightly funnier on TV than on album). Rolf gets his best piano-bar hack on for a duet on “Have Yourself a Merry Little Christmas” (Denver sounds like he's singing about ten feet from the mic). “Christmas Is Coming” is an utter head-scratcher, a Scooter/Piggy duet round with a weird island vibe. “Little Saint Nick” is as close to rock as we get, “Silent Night” remains orthodox and “We Wish You a Merry Christmas” very funny.