Another possible use of this is for combining multiple repositories into one. A few examples:
- http://www.gelato.unsw.edu.au/archives/git/0506/5511.html
- http://jasonkarns.com/blog/merge-two-git-repositories-into-one/
- Combining multiple git repositories
In these cases you will have two separate DAGs in the same repository before they are merged into a single unified tree. Thus this is not as much a long-term use, but an action that will temporarily pass through the state of having separate DAGs in the same repository.