I'm pretty sure all the various continents are on their own maps, and not a big mega map. That's kinda why you boards ships and airships and stuff, to mask the fact that your character is being moved from one map to the other. Not having fatigue wouldn't have any impact on your inability to fly from the eastern kingdoms to northrend for example.
Other than that...isn't fatigue pretty much there to stop us from heading out onto the empty parts of the map players aren't intended to access because they're empty and you could fall off the map/world and otehr such issues?
And if you're really that curious about what exists beyond the fatigue zone...aren't there programs out there that allow you to see the entiore zone maps and such anyway?
Seems a better option than the ability to aimlessly wander empty ocean just because its there!