What advantages do they give themselves and how does it affect multiplayer (non-zombie)? In the first Black Ops I only played zombies. There were lots of cheaters but I just refused to play any games with them.
Simply, they have an extreme advantage over other players with auto aim bots, wall hacks (they can see a square around you thru walls), and other hacks like auto knife. They can even create a delay to have it appear they are lagging and therefore almost impossible for you to hit. While not everyone takes advantage of these hacks, there are enough (even 1 or 2 in a match) to ruin the experience for those who don't use hacks. Matches can become one sided as that player gets killstreak rewards a lot faster and of course with the perks of the hacks themselves.