One of the strongest arguments is modularity. It's definitely true that you prevent your code from rotting and acquiring random dependencies if you think about how you can export it as a useful open source library.
As I and my teammate started working on Candy (http://amiadogroup.github.com/candy) we knew that we want to open-source the project. Therefore we built everything so modular and flexible that even we used the vanilla Candy and wrote just some custom plugins for our own use-case.
