Slopes, angles, suprise, deception, overall caution, acceptable losses, staged feints, they just won't have the impact with simplistic rules and a single mind on the other end of things.
Actually, they can be used, providing the GM is taking them into account, or the module is set up for that.
Picture d20 Modern, some fool PC group charging uphill, with no cover, two guys running an M-249 SAW (with the loader providing backup with his M-4) and shooting at them.
That's a damn deathtrap right there, but how many PC's would do it? Because most of the time, people don't both with bonuses/penalties on the actions.
Give the gunners a total of +4 to strike, no penalties to burst fire because of: A tripod for the SAW; prepared fire zone, complete with range cards and aiming stakes; firing downhill into fools standing upright; sandbagged position.
Drop the PC's speed, running, to only 40 or 60 ft per round, and make them fatiqued after running 500 feet up the hill. Even if they drop prone, they do NOT gain the prone penalty, since the elevation and slope let the LMG gunners see them sprawled out like roadkill squirrels. Then, firing on the move, uphill, they're going to take MAJOR penalties.
If we put the sun behind the LMG gunners, it's gonna get nasty.
You can use the standards of combat, as long as you write them down and apply them evenly each time. Plus, give the players a chance to pull locked in ambushes sometimes.
One thing about modern firefights, is that they are, in general, fast and brutal occurances.
Now, as to the original topic, giving them something worth fighting for? That's simple. Immeserse them in the world. Let them affect the world. Unless they can change the world, and make a difference, they'll never really give a shit. They have to give a shit about the world, the setting, the NPC's, and what is going on. They have to have foes that they can hate and respect. They have to feel they are part of something larger than themselves, and that they are making a difference.
It's tricky, but it can be done.