congratz on 1000 posts lol.
back on topic, Vehicles will obviously influence teamwork even on a smaller role. The part about friends is an area where the player must make the effort to get into games with their friends and keep a relationship going by seeing them. Many of my friends have only seen me once and that is both of our faults. In Socom 3 I am going to try and keep bonds with other players so that eventually, I will have a constant group of say 5 friends that all log on into the same room and work together instead of running around. It will be a heck of a lot easier to drive with friends in a vehicle than with other people doing what they want.
Friends are not zippers main concern, I think the main thing they provide is a way for friends to interact better in-game and have real reasons to.
I believe that until you know a person you won't trust them to do their part, so you will try to do it for them. It is up to you to know a person. Of course a easier to keep up friends list (which I have heard you can use in game) will help in this area also.