There are two options for putting terrain models together: append (which uses all points: to some extent, as though you're dealing with one point cloud) and merge (which uses only the new points).
Sometimes I'd like to have only the lowest points between the proposed and the existing. Possible applications include structure excavation for drainage--if your roadway is in cut, you want to use the proposed subgrade line for your structure excavation; if your roadway is in fill, you want to use the existing ground.
Sometimes I'd like to have only the highest points between terrain 1 and terrain 2.
So I'd like to suggest this enhancement to the complex terrain options: Add also "merge higher triangles only" and "merge lower triangles only".