In multiplayer it plays in to destroying or creating line of sight. You could for example blow the floor from under the enemy while they're shooting at someone else which totally screws their aim up because they're now falling a floor (or more) down (happened to me many times). Or you can destroy the cover the enemy is behind.
No. The multiplayer destruction is running of the cloud. There is no destruction in singleplayer.
Smoke and mirrors? Red Faction Guerilla has real time destruction. From what I've seen of CD3 destruction doesn't look any better or more detailed than Red Faction's, in fact, debris seems to stay on the ground and remains interactive a lot longer in Red Faction.
