That would mitigate the problem of easily finding out what options there are, but it would still be a form of weak typing where a best guess is substituted for

That would mitigate the problem of easily finding out what options there are, but it would still be a form of weak typing where a best guess is substituted for a mistake. If the system administrator truly intend for a value to be default, he should explicitly declare it so in the configuration file. Interpreting the system administrator's act of forgetting something as his intention for it to be a default value, seems like a bad idea to me. As for merge, I find it as annoying if software not support configuration directories. `foo.conf` should exist alongside `foo.conf.d`, and if the latter exist, it's contents should be parsed as well. On an update, the system can simply install a file in `foo.conf.d` which contains the default of any new keys added from the last version.