Spawn Trapping I can't really agree with as being an exploit, though I feel it is a cheesy tactic myself.
Think about combat zones. One aspect of Warfare has always been to cut off your enemy's re-supply and reinforcement routes. Same on CoD, especially in Objective Modes. You take the Objectives and cut off the enemy options. Why would, in a combat zone... reinforcements show up in an enemy controlled zone?
It's not an exploit, as good teams can break a Spawn Trap with coordination, and normally wouldn't allow themselves to be trapped in the first place.
Cut off the enemy supply lines, you win the battle. I know this is a game, and not real life, and spawns HAVE TO be improved, but the principle remains the same.