Matthew Weier O'Phinney, of Zend and the Zend Framework project, has put together a Storify stream of tweets he posted about some of the benefits of having multiple repositories in a project.
I've seen a number of critiques and write-ups recently about how monolithic repositories are intrinsically better for developing large projects than using a multi-repository approach. In the past year, we went the other direction, splitting our monolithic repository into individual component repositories, each with their own history, tests, and documentation. This is a summary of our experience.
He goes through a list of six different things they learned as a part of splitting up the (Zend) framework into multiple repositories instead of one monolithic one. He includes the contents of each Tweet and a paragraph or two giving it a bit more context and some examples of the changes that were involved. There's also a bit in there about changes they made to the documentation for the project as a result of these repository splits.
I'm quite happy with the switch from a monolithic repository to individual component repositories. I think our code quality is improving dramatically as a result, and I'm excited about the future of these various code bases.