Simple, I would hate it if the ground was completely flat

The heightmap also blocks a lot of distant shots, so you're less likely to get killed from a miss shot meant for another soldier in front. For the same reason, you can and need to get closer to the enemy, so the fights are supposed to happen within a small distance to make it more intense. This could be achieved with a flat map too by artifically reducing the weapon range, but it doesn't feel right.
Currently, in the camera, the darker terrain areas are lower and the brighter/natural colored areas are higher in relation to your soldier position. You basically need to have a Shader Model 2 graphics card to see it, as the effect is using vertex and pixel shaders. At least I find this effect helpful in finding a higher position to use M240 from to get more open range.
I could try different factors for the effect to make it more visible, if that's the issue. First, make sure you have the SM2 graphics card though.
Having said that, you can now argue how the height should be used and what should be avoided. The current heightmap is a random noise cloud with some intentional evening out for the buildings and lowered trenches, so the open ground has small hills and valleys here and there. Are those too steep, do you think smoothening them would help?
Camera movement is pretty much out of the question, you can't lower the camera too near to the ground as the soldier appearance is not suitable for close viewing.
You can try making the heightmap flat yourself to see how it would work currently. Just edit the media\maps\map1\terrain5_heightmap.png by filling it with single shade of gray, anyone should be ok, and let us know what happens. Of course, this doesn't reveal the whole truth about flat maps, as most of the balancing so far has been done on a lively heightmap and it should make a difference here.