I'd go so far as it being an essential feature for a navigation app. A bright device is only going to distract you, even on low brightness. With a black background and roads in say, grey, and the brightness down, the viewing isn't distracting.
I'd also like to see a "custom colour" option - here in the UK, Motorways appear on most maps as Blue, Major A roads are Green (also signified on roadsigns). It would be nice to see this replicated on Apple's maps. It would make the map appear a lot more useful.