So, to your point, "bringing in outsiders once in a while would be a value add.." and they aren't doing that for you (and you're paying them), wouldn't that indicate an opportunity to improve how you manage the CB relationship (since they apparently haven't stepped up to the plate?)
IMO, there's a pretty fine line here...
CB auditor is certainly "outside eyes". They also bring knowledge of how other companies handle similar situations, to the degree they can share.
The fine line is between the CB role and a consulting role.
CB auditor verifies compliance or non-compliance but, as repeatedly vented here on the forum, should not take the role of consultant.
There is certainly value having outside eyes measure against a standard. There is also additional value having outside eyes make suggestions of how to do things differently...even to the extent of "You should...".
CB auditor can add some value. An outside
Internal Auditor is in a position to add more. My thoughts anyways...