Anyone know the purpose of the pin that lines up with valve stem, and only allows rim to go on in one position (out of five) on a 5 lug rim?

Must be some reason, or they would not have bothered to do it. Just wondering what it is..

 

That pin is there for "on-the-car" wheel balancing. When this is done (it takes a special machine and is not popular nowadays) the tire, wheel, hub and rotor are all balanced as a group. In order to keep the respective positions of the parts, the pin allows the theoretical replacement if the wheel is removed. In those cars that use bolts instead of lug studs, the pin is useful for "hanging" the wheel on the hub so one can place the bolts more easily.

 

I have a couple of cars with the pin removed, and it's really pretty difficult to get a wheel on. The pin is an alignment tool for the other 5 holes. It would make sense that the reason there aren't 5 pin holes and only 1 has to be for on-car balancing