Right then. While I like the whole idea of troops being able to "dig in", I think what we need to realize is the amount of time and effort required to actually dig even a small shell scrape, (deep enough for an infantryman to fight from, lying down).
AGES... Thats how long. For example, to dig a 4 man (fireteam) trench takes a minimum of 24hrs. (unless the engineers show up with specialist digging equipment ie a JCB!).
The rules you propose are still valid however, as it could just represent troops taking more care about how and where they position themselves, taking advantage of every fold in the ground etc. Similarly vehicles as you say can benefit from the rule also, the vehicle crew just positioning their vehicle to maximise their protection, Hull down as primarch says.
I dont think that the positions in this case would remain on the table after the unit moves away though.
If you want trenches etc, then i think they should already exist before the battle starts.
As for the role of combat engineers, there are still some good uses for them, such as erecting bridges and clearing minefields, and assisting with assaulting heavily defended positions with explosive charges, among others. Perhaps the "combat engineer" could be an addition to an existing unit, ie add 1 stand of engineers?
Just some thoughts.....
Steve
