As a professor, I collaborate with other authors all the time. But I rarely assign collaborative writing assignments. I often ask students to critique each other's papers or beta-releases, but that's not the same thing as having them collaborate to create a single, coherent document.
I'm teaching a literary criticism class next term. The last time I taught it, students expressed a lot of frustration at how long it took them to get the hang of what criticism is. You're not writing summary, or personal opinion, or factual investigations about the author's life. So what do you write about? I can't really think of a way to get students to learn how to do lit crit, other than having them read models, talk about it, and try it.
Even though I always hated group work when I was an undergrad (and I never did any as a grad student -- not once), after a conversation with my colleague
Lee McClain, I think I'm going to have my literature students write their first few papers in teams. I'll probably have some mechanism so that a student who is part of a successful team paper can request to write the next paper individually. That way, the students who are sure that they can do better on their own can opt out of the group work quickly.