[Mustang]

Zipuli, the "clear" terrain modeled into the game isn't perfectly flat. There is still cover on the ground (small bushes, short hills, etc.) that infantry are assumed to take advantage of. So, because they're already taking advantage of this cover, making a very small trench like you suggest would be unrealistic.

About tanks digging in in SP3: Each SP3 turn supposedly represented 30 minutes of real time (according to the manual). It took maybe 4 turns to dig in, and I guess that you're supposed to assume that the crew climbs outside of the tank and shovels some dirt in front of the hull. Wouldn't be all that unrealistic, if 4 turns meant two hours of real time.