Assuming you are taking about a VFR joiner
I am. Thanks for the thoughts, all good points.
C) have half a chance of seeing the other traffic you are telling the joiner about
This is, I guess what puzzles me. Passing a VFR joiner from Approach to Tower when it has "half a chance of seeing the other traffic" seems like a sensible transition point. However, sighting a square mile of airfield is likely from a much greater range than seeing a 30 foot plank of a wing with a small fuselage attached.