Good comments, all.
What prompted my posting of this topic is the Half-marathon and Marathon courses that are primarily gravel roads. There are a fair number out here, but maybe in the east there aren't as many gravel roads. I bet the midwest has lots of them, as there are few paved roads outside of towns.
But, on a marathon course, if there is 1% slippage, the course could be 421 meters long. That's a noticeable difference, and would justify GPS-wearers in saying "the course is too long". I'll be interested to hear from more people, if they have an opinion. I would like to agree with Pete, and lean towards 'leave well-enough alone', but I just wonder if the long courses warrant further discussion.