Having a sepearation of ranked playlists and social meant that you had one playlist you could go in and know that people on your team would all be trying to win, at least at the higher levels, so as to increase their rank. Losing caused your rank to go down, and people playing in that playlist wanted to win to avoid that. Then you could go in social and just mess around and not really try to win. WIthout the visible ranks, a lot of times your team has people trying to win, and others just messing around (i.e. Reach). If you want a team of people using mics and all trying to work together and win, you have to have your own team and definitely can't depend on randoms.
If the skill level is there, but invisible, it seems like there's less incentive to attempt to maintain that level. Sometimes I just want to have fun, other times I want to be on a team and try to use strategy to win.
And boosting as being their reason for not having visible ranks is pretty lame.