K/d should not influence rank.
If you're thinking k/d per game, than there are many times were k/d must be sacrificed. Even in the pro-circuit, the winning team will have negative k/d's, especially in objective based games. Also, it is very difficult to carry someone with little knowledge of the game to a victory once you have started playing people of higher ranks and familiarity with the game. Furthermore, this would encourage more 'kill stealing' (not that I believe there really is a such thing in a team game, the win is most important), but who wants to have that guy who sits at the back of the base with all the sniper ammo the entire game, apposed to moving around and being active with the weapons and engaging in the game.
And if you're talking about k/d over-all, that would have a devastating impact to the already dwindling community (if you can call it a community in Reach). That would discourage new players who are trying to learn and are dishing out terrible k/ds as they getting use to the game. Once they gained they're skill they may have to fix something discouraging such as a .60 k/d and 15k deaths. Whereas the gain two rank up and lose two rank down lets a player grow while still basing each other off of the current and over-all skill.