Well, now that my super-secret playstyle is out in the open I might as well speak on it.
For repairs, you get one Mission Point everytime you "build" something - ammo box, turret. If you destroy an enemy's ammo box or turret, you have the ability to build on top of it and make your own. I'd say, on a normal sized map, there are 10-12 spots that are able to be built/repaired (easy 10-12 points off the bat). You can't destroy a teammate's spot and build your own and get rewarded with a point, and you don't get a point for fully repairing something that's already been built (i.e. someone else's ammo box, turret, or bot). Most Mission Points, I found, come from going back and rebuilding an ammo box or turret once an enemy has destroyed it - and it's very common for me to have people constantly blowing up my shit around the map. It's not just something you do at the beginning of a game and leave sitting there, you have to constantly run around and make sure every spot on the map has an object there. In the end, I'd say having a repairman on your team is beneficial to the player (points) and the team (ammo, turrets).
When you combine that with the ability to rack up mad points killing people (50 kills a game or so) then you get loaded with points and are always at the top of the scoreboard. Also, my preference is to use the Medic class with this ability. I get the 1 point for reviving a teammate, plus it sort of makes me the support player on the team (the fact that I bring people back to life and provide them with turrets and ammo).
When I'm playing alone, I often get more points than the #1 person on the opposite team who even gets the 1.5X for winning. My best match under normal time limits (as compared to the current trend of making longer games that last ~30 minutes) was 149 - 68 kills, 11 deaths, and 51 Mission Points (223 with the multiplier). However, as I'm usually playing with at least one or two other good players, my average for the normal ~15-20 minute game is about 110 with the multiplier, depending on the map. Although I usually only get a good four nights in each week with Killzone 2, I usually get 4500-5000 points a week for a small amount of time.
I wouldn't advise using my strategy if you can't hold your own in the game or have a shitty K/D ratio (as you'll likely be traveling behind enemy lines to get in some repairs and/or heals). Also, whenever I run into another person on my team who uses my repair gimmick, I notice that they only know one or two spots on the map and they usually stay on the complete other side of all the action. Don't do that. For one, you won't get points. Secondly, one or two people in the game can make a big difference when you're going for a bomb arm or propaganda return. If you play with the Repair ability enough, you should be able to see a nice line between where you are and the objective that allows you to repair shit on the way to assassinating a target or defending a position, etc. It's almost like connecting the dots, except in this case you're racking up points as you move through the map to complete whatever objective. Learn the maps and all the points and you'll be swimming in points before you know it.