Capn Bloggs,
You are right. If a clearance cannot be issued by the controller, then it is for a reason. Most likely it is workload, traffic or frequency congestion. In all cases, if the controller is unlikely to be able to give a clearance, they are also unlikely to be able to pass traffic as they are required to do immediately on receiving your request.
As an ATC I have seen/heard of 3 times IFR pickup being used. In two cases it was an inappropriate request (G going into C) and all were many years ago after it first came in.