I encourage duet playing during my lessons as it is a great way to develop listening skills between budding musicians. It also teaches the importance of counting carefully.
Duets are a great way of introducing ensemble playing. As a beginner pianist ensemble opportunities are more difficult to find and the duet or trio can provide a similar sense of enjoyment, therefore I often play duets with my students and also enjoy pairing up students of a similar ability to play duets.
Duets also work well for the ensemble performance module of GSCE music and I am very happy to prepare a duet with this level of student and also to perform with them for the recording on the day.