I thought I read once they unit coins started in the Army. SOF teams in Vietnam had them made and gave them to the indigenous soldiers they trained and fought with to help them identify them to other SOF teams. I'm not sure when the Air Force started using them, but I can see how the tradition can now been seen as a frat boyish thing.
I've always bought coins for the units I was assigned as a matter of unit pride. If I got some from commanders or other units for a job well done, thanks, but I don't put much stock in them. I haven't used them as true challenge coins for alcohoic beverages in about 10 years.
I remember in Korea, some shops sold extras of unit coins from all over the world. I saw some guys going in there and buying coins which I thought was weird. If it's not from your unit or from a GO received for a good job I don't see the point. But, then again collecting military memorabilia is a big hobby/business for some.