The problem with the split is that it forced smaller lists to make a decision: were they social (and have potentially more players, but come off as slightly more relaxed) or ranked (and have less players, and percieved as more serious)
It also has implications in-game and was demonstrated with the unranked -> ranked -> unranked Halo 2 SWAT playlist. When H2 SWAT debuted, everyone just played through the games, churn was high, it was all good. Then H2 SWAT was ranked. The number hanging over everyone's heads suddenly had a noticable change. When you now played games, people turtled hard, and when the games got near the end in time or points, people would hide and wait for someone to get bored and run out to try and get them.
The population eventually dried up to the point that H2 Ranked SWAT had to be deleted. It was reintroduced some months later as an unranked list and had a healthy population until Halo 2 was killed. And really, for Halo 2 the ranked/unranked was used more as the toggle for "no guests / guests". Halo 3 was the same way. Reach has guests/no guests as a completely separate flag they can apply.
And you can't just go making an unranked/ranked version of every list, because not all lists support doing that. It splits the potential population down a certain line, and could be fatal to what would have otherwise been a viable playlist. For example, an Arena or Ranked Objective would be dead within the first month, because you'd only get the more hardcore elements out of the existing TO list. There isn't tons of potential TO players out there waiting for a ranked list to show up and play.
edit: also it looks like I should finish Gears 3 before I get spoilt
