Why? Because I once walked into a pretty dangerous ghetto in Memphis thanks to having a map leave out information (even though it was relatively to scale).
Here's the map for the Stax Museum: http://www.soulsvilleusa.com/plan-visit/maps-directions.asp
And here's the Google map of the same area: http://maps.google.co.uk/maps?f=q&source=s_q&hl=en...
OK, so the Google map is way busier and contains a lot of noise. But because of the Stax map we chose to walk from the National Civil Rights Museum to the Stax Museum because "the map shows it's only a few blocks over".
That few blocks turned out to be a 10 block walk that involved police (who turned back once we walked beyond a certain point), watching a gang trying to cut us off (they were armed), being rescued by a kindly lawyer who then drove us out of there and back to the police. Apparently walking through that neighbourhood is a death-wish.
The Stax map isn't of poor scale, it's just that it's aimed at drivers following main roads... a bit like the napkin map. The problem is, you cannot be sure of every use of a map and it's safer to leave more info in than risk decisions that will be made because of a lack of information.