I disagree. A kill means you stopped them before they killed you. If everyone on your team had a positive k/d ratio, you would win. Almost guaranteed. So score DOES matter. Going 0-4 in 15 minutes top lane vs an akali MATTERS. Now, let's say you're on the receiving side of the feeding. You get a **** load of gold, exp, which translates into more ad/ap/health/mr/armor, which translates into a stronger hero which normally translates into more kills.
There are some situations where the team with the higher amount of deaths wins, but it's not often. Kill them, before they kill you. That's how you win this game, like it or not k/d matters.
Also, you don't win this game by killing them, you win this game by killing their base. One often leads to the other, but there is definitely a difference.