People see K/D as an indicator of a players skill. And since they don't have a better stat they fell it matters. Personally I think that K/D only matters in TDM. In ever other game mode, even FFA, W/L would be a better indicator. If I win every game of FFA but others have better K/D in FFA who really is the better FFA player. Same goes for objective based game modes. In team based games however W/L is skewed by players only interested in maintaining great stats. And many players don't have control over who they are teamed with. So although W/L would be an important stat for FFA, the team aspect of objective based games which is not simply to kill (like TDM) makes it a unreliable stat as an indicator of a players skill in objective based team games.
In TDM a positive K/D generally means a positive contribution to the team goal, which is to win by killing more than you are killed. However, like in many of the game modes, this can be abused if some of the players don't really care about winning as long as they maintain positive stats. I have never understood this mentality but I know many care more about there K/D than winning the game. They see it as an indicator of their skill because, although not a perfect indicator of skill in TDM, there is a high correlation between K/D and a players skill in TDM. This is why I have suggested a better stat for TDM, a weighted K/D. Specifically I would like to see a stat for K/D x average kills per game (lets call it KDA). This would be a far better stat in TDM as an indicator of a players skill because it also take into consideration their contribution to any victory. In my case my KDA in Ghost would be 2.09 x 15 = 31.35. Of course even this stat can be skewed by players who only play in a team or leave when they aren't doing very well but, in spite of those limitations, it would have an extremely high correlation with a players skill in TDM.
For objective modes I think many of the problems could be resolved and the stats more reflective of player skills if IW implemented a 2 tier point streak system, one for objectives (which hurt your personal stats) and one for kills. Objective streaks would be rewarded with assault streaks and kills would be rewarded with support streaks. It also balances the K/D better by rewarded players that play the objective and risk their life with streaks than obtain kills. See IW you can use other mechanisms, other than nerfing everything, to balance the game.