Well, in Killzone 2, there are checkpoints. You can't go back to certain zones once you clear a checkpoint (doors magically close, by example). Maybe Helghast are not spawning eternally, but they are spawning at specific places and you can't progress unless you kill them all. Case in point, in the first mission, the collapsing building. Maybe I have the wrong definition of scripted, but I'm finding there's a lot of this type of events to spice the gameplay. Sometimes it adds to the game, others it feels too, I dunno how to express it, artificial.
Another brilliant thing going on for this game is the hit detection system. I really enjoy how the Helghast (or even ISA) are going down after a couple of shots. Pretty neat stuff.
On the topic of cover in multiplayer, I totally disagree with Guerilla. Gameplay wouldn't be the same if I could peak from a wall instead of running in front of a ennemy and give him an opportunity at killing me before I see him. It's like saying the mp in Gears of War would have been the same without the cover system seen in its campaign mode.