You gain bonus points for planting and defusing the bomb.
I know a player who has a K/D ratio well below 1 who is very highly ranked due to the fact that he concentrates on completing the mission rather than killing dudes.
So the more you defuse the bomb the more points you gain (Its about the same as killing 2 people).
I've written a "points and ranking" guide which was submitted to the Spawnpoint "Gods" and, hopefully, when they feel up to it they will put it on the FAQ's page so EVERYONE will understand how ranking and points work
Rob