My piano teacher is big on duets. Probably because they really require players to keep correct tempo/rhythm -- and to *listen* to each other, as Cas pointed out. Besides, they can be great fun for players and audiences, as everyone's pointed out.
She's mentioned having her younger students play duets at recitals ... but she also gets us oldsters into them at the piano parties she hosts twice a year for us. She and I have played together a couple of times. While I can't say I was thrilled with my performance either time, it was helpful to get me started as a performer. At this December's party, she played a duet of "Sleigh Ridge" with one of her (very) advanced high school students, and we were all surprised when my teacher suddenly made a big mistake and get lost in the music right near the beginning. But she just said "See? Everyone makes mistakes," and they restarted the piece, which went perfectly the second time around.
A couple of weeks later, I had the very great pleasure of seeing/hearing her play as one of EIGHT pianists on FOUR pianos at her church's "Keyboards and More" Christmas concert. Fantastic!
(FYI, the pianists were the stars of the show, but there was also a small orchestra, a father-and-sons saxaphone trio, and a harpist who performed her own stunning arrangement of "Little Drummer Boy" -- drumming on the instrument as well as playing it. Amazing.)
I'm starting to really groove on the idea of playing more duets. My teacher did mention something the other day about the arrangement of "Silent Night" I've been working on having the secondo duet part written in below ...