Quite frequently, custom fields have a specialized use and you don't want to allow the majority of users to edit them. Just like you can set policies for "Can Edit Task Projects" or "Can Edit Task Status", it would be useful to have "Can edit (custom field)" for every custom field in use.
A beneficial side effect of setting this policy is that we could keep a simple and clean form for the majority of usesrs to create new Maniphest tasks, leaving i.e. "Story points" only to those that will do something about them.