I was timing a Thanksgiving 5 km today. After the race the director (who was in her first year and not a runner) asked, "If everything in this country is measured in miles, why do we have 5 km races?"
I couldn't think of a good reason for that. I do know, however, that when I've consulted on new events the organizers universally feel 5 km is a more popular distance than 3 miles (I've also seen more than my share of great 3 mi. courses screwed up to work in the additional 188 yds. to get it up to the "magic" distance, but that's another topic).
Anyone have any thoughts?