The way I see it, ADServer needs MySQL just as a configuration and project code/data storage. Afterwards you are free to register any other server for your scripts.
So, why doesn't ADServer come with a bundled Apache Derby by default? This way you reduce a large dependency and configuration step.
Aqua Data Server uses MySQL to store meta data about users, permissions and projects. We needed to choose what data source vendors we were going to support. We needed to keep it down to 1 vendor for version 1.0 so that we could minimize development. We had the choices from Apache Derby to Oracle. We didn't want to force a dependency on a commercial product as that would increase cost of ownership. We also didn't want to choose something that had little scalability. So, we chose MySQL as it was no-cost and scalable. In the future we may add new vendor support for the meta data store based on user demand.
Aqua Data Server uses MySQL to store meta data about users, permissions and projects. We needed to choose what data source vendors we were going to support. We needed to keep it down to 1 vendor for version 1.0 so that we could minimize development. We had the choices from Apache Derby to Oracle. We didn't want to force a dependency on a commercial product as that would increase cost of ownership. We also didn't want to choose something that had little scalability. So, we chose MySQL as it was no-cost and scalable. In the future we may add new vendor support for the meta data store based on user demand.