Well, I mean, you would know some of the land of course, but i'm saying that in big maps, I doubt the dwarves would explore it all, just enough to feel safe.
Thats all i'm really suggesting, It would be nice to have to explore some of the map on occasion.
In general, I don't think the maps are big enough to warrant the minor addition that it would be. You should be able to get a decent view of the land that, during daytime on a high place, you can get a general idea of all the features over the map. It's true that the detail wouldn't be perfect (and I'd be more tolerant of the suggestion if it would still have a way to display large features without going into perfect detail), but you should have a much better view of it than the normal game treatment of fog of war has (vision just stopping at 30 ft or so away.)
To do that at the surface is basically a trivial thing that pretty much anyone will do within their first few minutes of starting up a new fortress unless the terrain/creatures are hazardous. It also requires the player to draft and run some dwarves around, which adds an extra pressure when starting up the fortress. If it's required, it'll frustrate newbies and annoy people who've played for a while; if it's not required, I guess I can't really see it being beneficial to anyone. If people still want it, I can't see a simple init tweak like [START_SURFACE_OBSCURED] taking too much effort from Toady, though.
But, let me say that I'd be fine with something similar to this for when the army arc comes in and you're positioning armies around your nearby land (and possibly underground) for maximum effectiveness. In my conception, that's at the point where dwarves generally won't pre-emptively explore or pretty much already know the general layout from having two functioning eyes. Of course the size of the embark locations will vary, but that can be construed as the boundaries of your fortress's zone of direct control.
Having all outdoor tiles revealed to begin makes sense, and really, I don't even think it's all that bad instantly auto-scouting underground features...
but I'd really like some kind of fog-of-war that affects creatures, along with some kind of look-out function. Ambushes makes sense, but invisibility that only disappears if you attack or come adjacent to a creature is a bit much.
Dwarves have a spotting skill in the upcoming release. I'd certainly be fine with the idea of having lookouts atop towers to be able to see nearby wildlife/incoming armies/ambush squads even without sneaking in either a forewarning before they enter the map or to be able to see them on the map, but I do think the terrain should not be obscured.