In the Call of Duty: World at War single-player/co-op campaign, enemies spawn in an infinite stream until the player pushes forward to seize the objective. That means that how many you kill is irrelevant. They keep coming until your player stands in the targeted location.
Medal of Honor: Airborne does it differently. If you stand in one place and keep shooting enemies, they'll continue to spawn for a while, prolonging the experience. But the game eventually rewards you for your kills by ceasing to spawn new enemies for that location.
These are different kinds of games -- one involves pushing forward on a linear path, while the other lets the player parachute onto any location of the battlefield and approach objectives from many angles. But regardless of that difference, I'd say Airborne's spawn method is better. Even if reaching locations or destroying AA guns (or whatever) is the main goal, kills should be rewarded.
Every activity central to gameplay should be rewarded. No accomplishment should be completed overshadowed by another.