Displaying it would be a pain. Determining how high creatures are vs their surroundings can be a pain too (eg. a dwarf stands on a square with height 2 while adjacent to him is a silk sock on a square with height 5. Can he grab it?)
Besides being a logical step, it carries a lot of disadvantages, not the least of which is the hassle of updating almost every game interaction. And if that doesn't happen it wouldn't be worthwhile anyway.
For brooks there are other potential solutions, like using the floor cover tile on its own as a floor tile, without requiring water to be under it in worldgen. In addition, ramps could be considered half full instead of completely full, so they too can serve as shallow start points for brooks.
Lastly, check out the
Depth by Darkening thread. When that is in, you'll be able to see and enjoy more of the already existing terrain. There is terrain variety now, it's just poorly displayed.