You can't have a competitive game if there is nothing to lose.
Think back to when you would play halo 2 and halo 3, and it was 45-45 in team slayer. Think about that adrenaline rush that was associated with each kill and death from that point on, knowing that if you lost, you could rank down or if you won, you could rank up.
Your team coalesced because you were fighting for a common cause.
Now what happens in Halo reach? You are lucky if the match even ends in 4v4, and there is no ranking based on skill (because so many people quit games and flat out don't care.)
TruSkill is flawed in some ways, but it was fun.