Currently FF (and also TB, indeed) support policies via a single policy.json file; in this way if a configuration management system need different configuration for different (set of) host, it is forced to prepare a different 'policy.json' file for every (set of) host.
It is true that modern configuration management engine support direct in-file editing (take Ansible as an example), but this is a bit 'clumsy' and can lead to trouble (if the editing corrupt the json format, all the policy get discarded).
Also, 'split file' is a modern way of configuring things, see Debian distro that use extensively split file for their configuration.
Consider an example: i need to install FF in my organization; management sector need plugin A and B, production sector need plugin B and C. I need to setup three json file: the base one without plugin, the 'AB' one and the 'BC' one.
A better approach would be to have FF to load policy as 'split file', eg load all the json file in a particular folder: in this way i can check json syntax correctness before uploading the file, and FF policy parser can reject a single json file, not all the policy.
Even direct in-file editing will be safer, if needed, because i can modify a single file and not all the policy.
Staying in my example, in this way i setup the base policy 'policy.json' and the plugin policies 'A.json', 'B.json' and 'C.json'.
With this setup i need only to copy 'policy.json' to all computer, and copy the plugin policies file where needed.
I hope i was clear.