I'd say multiple landscapes connected at the edges are a simple hack to work around engine limitations and/or to save the artist the work to do proper transitions. On the other hand, putting two locations which are far away from a story point of view into the same landscape is also a hack, but one which could be made indistinguishable from the real thing with only a few extensions. Clipping the viewport to a subset of the landscape, creating a material which acts the same as the real landscape border (where it isn't vehicle) and changing the sky picture is about all that's needed.

If they can be distributed over the net, that could lead to interesting possibilities for permanent online games... (if something like that is still on the engine-devs wishlist ;))