On one side, you have realism. You need at least a bit of it, else the Players won't be able to suspend their disbelief and enjoy the game. On the other side, you've got playability. Plainly put, this game system is concerned with how useful your combat unit is and not exactly who got shot in the leg. Vehicles most often blow up when hit by weapons fire.
Infantrymen become casualties.
Sometimes a shot isn't strong enough to blow up an opponent and just cripples him. No, you're interested in whether or not he's still a threat to your mission. Thus, the principle guiding this entire system is functionalism. Most hi-tech gamers love vehicles and other mechanical constructs. Regardless of the type of campaign they play, they want cars, boats, starships, giant robots.