From what Lee told us at the convention, its a question of performance.
A game like Battlefield 1942 will require very different optimisation techniques to a game like Quake or Doom which is primarily indoors.
FPSC's system works on drawing only what is visible from a given "room" (an area of space). With a terrain, you would probably need to draw everything, but reduce the amount of detail considerably at a distance - the "Serious Sam" approach.
The other issue is that FPSC's editor is designed for construction of Doom-style games, and so will not have all the features you might expect in an outdoors-game editor.
You will certainly be able to create terrain segments though, perhaps Lee / Mike could integrate terrain support in a future version?
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