CB can be used to remove mandatory global parameters.
Just an example: what happens if someone sets and after deletes the valid-lifetime parameter. In a subnet (or a shared network) this parameter can be NULL / unspecified, its get method uses a ALL inheritance so in fact relies in the existence of a global built-in value.
This built-in value is done by a default value in GLOBAL4_DEFAULTS so using a configuration file or a config-set command we can assume there is always a default value. The config backend can change this and I am afraid it can be used to remove the global default.
Note it is a formal correctness question i.e. I do not expect someone will do this in production but it must be tested and eventually fixed (if possible without having the default value at 2 places in the code).