We guitarists spend a lot of time playing music by ourselves. It is way too easy to play rhythms incorrectly and not realize it, to stretch beats to allow for difficult movements, to flatten triplets into eighth notes, to unwittingly add a beat to the measure. Metronomes notwithstanding, rhythm is usually the first victim of sitting by yourself with your instrument. Chamber music requires excellent rhythm. If yours is incorrect, then your part won't fit your partner's part and the result will be a mash. So, first, can you play correct rhythms, in time?
Second is being able to play from a score while listening to the other part. This is a big challenge: it is quite different than focusing laser-like on your own part, and just hoping it all works out! The results are also quite different. So I have first semester students (who've had little or no prior ensemble experience) play in multi-guitar groups to hone these skills some and demonstrate an ability to play in time together, and play and listen at the same time. But then we move on.
By the second semester, its all mixed chamber music. Don't get me wrong: I appreciate the terrific repertoire out there for two guitars, and I do encourage it from time to time, as long as students are getting a healthy dose of mixed chamber music as well. (We do very little guitar quartet playing.)
So they accompany singers, flutists, clarinetists, violists, percussionists, trumpeters, cellists, recorder players, oboists, violinists. They are forced up a level, and do better prep, just because they do not want to be embarrassed at the first rehearsal. They have to realize that when a singer or wind player takes a breath, it is a physical experience, taking time to complete, and that, yes, the music will wait. And then later, when we are working on solo rep, I can refer to this experience to bring real musical breathing to their playing. They accompany a violinist and have to learn to hear the change of direction of the bow. Violinists, further, experienced in the major orchestral and string quartet repertoire as they tend to be, can bring a more refined sense of style to the table in the 19th century works. They'll understand exactly what articulation and phrasing is appropriate without thinking much about it, and wonder why the guitarists, on balance, don't. The guitarists are dragged up a notch or two, and play better as a result.
The rhetoric of classical music can't be learned by reading about it in books--it has to be played and heard to be understood. This is the most effective and direct way I have found to draw my students more fully into the world of classical music. They hear it, they follow suit and, by imitation, play with correct articulation and phrasing, dynamics and metric sensibility. And then, all those other benefits start to unfold: they are drawn into the broader musical community, they build more varied friendships and are better known by a wider faculty and so on.
And then, finally, the advantages for me: lots of fantastic music, a more varied sonic experience, interaction with more members of the musical community, the appreciation and respect of my colleagues for my interest in their students, and for constantly introducing them and their students to new and interesting repertoire, and finally, better audience turnouts, as members of all those other studios (and their families and friends) show up at our Ensemble concerts in addition to the usual guitar lovers.
Having the students play mixed chamber music leads to a situation I'd call win-win-win.