An analogy can be made between the 2d walk map and the collision mesh. The collision mesh would allow for better movement in a true 3D environment. Just simplify (viz. remove) all detail of the wourld mesh, and add in walls where you don't want the player to go. Test world collisions against it, but draw the world mesh.

Which method is best depends on the nature of the game. Dungeon Siege maps are put together like a puzzle from many tileable nodes. There is no collision against most environment entities (trees and such). And there is no true collision with things like building walls. Instead, there is a narrow band of unwalkable polys that border walls. Since the no walkable poly ever meets the wall, you can't go through it.

The collision mesh wouldn't work well for such a pieced-together world. It's better for maps modelled as a solid unit, and I'd imagine it to end up looking like a network of canyon-like tracks.