The convenience of not having to switch maps when moving to different areas of a city?
I can see a map like this being quite immersive for players - they can practically explore it on their own, and you can run scenarios on a scale that would otherwise be intimidating in a paper-only setting, such as an epic chase scene through a city, pre-populated with peasants and other random encounters (stats stored on the tokens are readily available when the players encounter them, so the DM doesn't have to keep a stack of possible NPC stats ready and look them up when needed).
You would probably still want separate maps for "key indoor locations" (temples, halls, sewer dungeons, etc.), but I can see the appeal of being able to switch scales easily, something MapTools can handle fairly well.
I can also see getting lost fairly easily, too
, which is why Phergus' suggestions for Wiki: goto()
macros would be a great idea, and excellent examples of Campaign-specific macros that have nothing to do with a rules system, and everything to do with the specific campaign and map.