All default values are in the core, so that only the exceptions need to be added to the app config file.
I didn't mean to say you should now modify a core file. You shouldn't, add your default override in the app file.
The reason for this is that there is a significant speed improvement when all config values used are actually defined. Which wasn't the case when the config was in app, as experience showed that while most people upgrade the framework to a new version, they don't verify if files in app require upgrading as well.