The reason kills don't give many points is because a frag isn't really worth anything for the team. All it does to sway the balance of the match is... well, nothing. The best you can hope for is to delay an attack but that's not exactly a titanic accomplishment. However, if you destroy a building, you seriously and permanently damage the opposing teams battle capacity. That's why a building is worth 750 points. Also, it's much harder to destroy a building than any unit. Only one character class on each team can do it real effectively and even they need 30 seconds. Any class can kill infantry units with ease (heck, I've taken out Mendozas with Hotwires).Also, makeing buildings worth mucho points promotes aggressive gameplay. That way you can't entrench yourself in your base and hope to win - you have to attack. If frags were worth a lot of points we'd be having a lot of stalemates. Now stalemates only happen once in a while.