Currently the mongod server upon seeing a cursor that is already locked returns a empty result to the client with a cursor id of zero. This will cause the client to consider the cursor exhausted and stop any active iteration.
There are cases where having multiple threads/processes reading from the same cursor is advantageous. The current processing makes handling those situations more difficult as the client cannot tell if the cursor is really exhausted or was just being actively read by another process.
Implementing this functionality would be a component of allowing clients to use a cursor as a shared work queue.