Well, in Star Wars: Battlefront II I was able to defeat 60 guys by myself, and once I killed 30 with an armed hovercraft shooting at me and no ammo. The guys kept going into the turret on the hovercraft and all I had was my overheat pistol to constantly kill them as they came in. I also used the pistol the whittle down and eventually destroy the hovercraft, jump out, get ammo and kill the few that were left.
I Dawn of War: Dark Crusade, I put buildings all over the map on single player so that when they attack, they only get the excessive number of buildings I have, and I get lots of resources and unit production to being with.
I'm sure I have done that before more in the example you mention, maybe in Command and Conquer or something. Game developers shouldn't have such blatant oversights in their games.