When assigning a user as an assignee, to ie an idea or feature, you can pick whatever user existing and even add a new user and assign, no matter whether this user does have contributor permissions on that very workspace the idea is assigned to. This bears the risk that the user assigned to ie an idea will never be able to make necessary changes due to the fact that permissions are missing.
inefficient processing and unnecessary delays in review of ie ideas due to the fact that you can pick whather user without taking into account permissions and thereby responsibilities on that very workspace
filter list of users available for pick as assignee to the users having contributor permissions on that very workspace (at least at point of time of assignment)