I think my interest comes from a slightly different angle.
So far I think people have forgone realism in favour of believability. Something I wholly agree with. Publius's example of the skyscrapers he thought would be gone in a thousand years is a good one. (He conveniently ignores that his fave idea, the fast food joints, would have vanished even earlier - though maybe not the french fries ) He forgives the fast food joints their unrealism because he likes them. I think the map has to give you the elements that encourage you to buy into it.
If I draw a street as 28 doors attached to 28 10x10' cubes. Each cube had better have a similar size thing in it or this should be a very unique street. If door one has 30 Bugbears in it, my players are going to either feel amused or cheated. The difference is in whether they have understood and accepted that the map as absurd - have they bought in? If they have, its a great map. So the second door has a Roman Trireme and the third has an infinite number of monkeys with a first draft of "Merchant of Greyhawk". This might work, once, but it has a limited currency.
Some level of believability avoids jarring threats to the players enthusiasm. I don't want to ask them to buy in again too often. To keep their belief its best if I give them something that excites them or that they like. Manga fans see nothing wrong with importing manga characters into Faerun because they like manga. Forgotten realms purists will rebel in horror. What the players want to believe is as important as their philosophy on what is believable. Enthusiasm over correctness - Happy over "right".
But I see another benefit of striving for 'realism', I'll call it plausible extension. This is a huge bonus to creation because it helps me choose what actually is going on.
If I have just drawn roads connecting a bunch of towns, plausible extension says that there is traffic for those roads. If I can extrapolate, from something in the map, how much traffic and where its going I can use that to write setting. If I am very lucky my players will make some of the same observations I do and they will say "Hey this is believable" because what I'm seeing makes sense to me. Or even better, given the map, they will do something smart to achieve success - thats a good RPG moment.
Plausible extension, or extrapolation, is one of the huge benefits of careful map making. Because you draw some of your story from the details in your map, having a logic in its construction can pay off. If you have a city with too little land to support its population, disrupting its food supply might be a stressful game element. If you can actually measure distances and travel times according to your maps - you can build strategies and plan trade routes.
Of course you can still pick an element and simply choose to make it a reality. Some players will not care or might draw enough important detail from the narration or some other element of the adventure. It depends on your style and that of your players.
I love Handsome Rob's attention to detail in his Atlas maps. You can daydream you way around in them and think of how the various elements might interact.
Personally, I want to be able to make maps with that level of plausibility and realism.