Dick,
1. Can you please provide the data to support your statement that ...
".....the Class D would immeasurably drop in safety when the single controller in the tower was attempting to control circuit traffic close to the airport as well as separating an IFR and VFR enroute aircraft possibly twenty miles away in the C airspace above."
That is , on what occasions has there been a safety issue ... or is it just your "best guess"?
2. Your statement ...
"...Derr' It's more resource intensive because a controller also has to separate VFR from IFR in C !"
may sound valid to the uninitiated.. that is, of course, unless you are a Controller and understand that it is quite often safer, easier and quicker to positively separate a VFR from an IFR ... rather than guess where the VFR may go and continuously vector an IFR (hopefully) around him. That is, assuming you have the surveillance capability to know where both aircraft are. And quicker means you can move on to another task.... hence it's less resource intensive.