Evergreen is open source software, which means you are free to modify it and distribute your modifications how you will. However, this doesn't mean that others are forced to use your changes. The canonical community version of Evergreen available from the open-ils.org website is maintained by a group of core developers and stakeholders who collaborate via the public mailing lists and the public IRC channel, both of which are archived. They vet the changes that make it into this version of the software and control the release schedule. Ultimately, they answer to the needs of the community, because the community is empowered to take the software with them. It is our hope that if you do modify Evergreen in a non-cosmetic way, that you submit your changes here to the community so that they may be adapted and shared for the benefit of everyone.
As with evolution, there is no final destination, no pinnacle of perfection, only a journey of adapting to the environment, the needs of the users. Evergreen was built with evolution in mind. Like every other well-written application, Evergreen is modular, but Evergreen has a service oriented architecture that makes modularity scalable. As your libraries grow, Evergreen will grow with you.
If there is functionality that is not here that you'd like to see, or that is here but you'd like to see sooner, let us know!