The current rank display is mostly described by:
Casual rank has a ladder and is displayed if it exceeds MM rank.
MM rank is merged into one ladder.
I'll explain the reasons for each.
Merged MM Rank
Matchmaker rank is merged because analysis by DeinFreund
(and possibly a few others) says that merging team MM games with 1v1 MM games yields more accurate winrate predictions than running a separate ladder for each. I agree with this analysis but disagree that accurate winrate prediction is the most important aspect of a rating system.
Casual and MM rank would be merged, or at least affect each other, if I had not fought against the desire for prediction accuracy that is gained from merging the ladders. Merging these ranks would annoy people who wanted to play big teams games without losing 1v1 rating.
There are other good reasons for merging 1v1 and Teams MM:
WHR is a hungry beast that, from what I can tell, uses a lot of server resources and sometimes breaks the server through unsafe commits or just taking too many resources. Making another rating sounds like it requires a whole extra instance of WHR resources.
There are barely any Teams MM games. It would essentially be impossible to get a balanced game because almost nobody will have been placed correctly.
Placing at the top of Teams MM would be quite random and be a cheap way to get Purple and further confuse the rank badge.
No FFA MM ladder exists because FFA MM doesn't exist. I am pretty sure that FFA MM should not exist.
The casual ladder exists and has an impact on rank badges because a lot of people would be annoyed if it didn't. Changing the rank badge to only depend on playtime and MM rank would essentially remove most people's ranks. You might say that they are wrong to want a rank and also want to mostly play large teamgames, but that is still what they want. It's a chicken and egg situation.
The rank badges are also quite useful in large teamgames whereas in MM they are mostly for bragging rights. Rank badges do the following thing in teamgames:
It tells you how experienced your teammates and opponents are.
It tells you the approximate competence of your teammates and opponents.
It tells you teammates how skilled and experienced you are, which could determine whether they listen to you.
It backs up some of the claims of the balancer. With no rank players could assume they have lost simply because they recognise the names on the other team.
Even if you argue that your opponents should be anonymous in teamgames, the rank badge is still very useful within a team. How many relatively highly-ranked people place their start position with an eye towards spreading the skill of their team somewhat evenly across the map? I would guess quite a few.
Just as a matter of fact, I don't think anyone can seriously argue that the custom game ranking is meaningless or incomprehensible.
Edit: Read this thread http://zero-k.info/Forum/Thread/29354