I can't quite tell you why it was implemented like this. The Omega paper by Google describes different kinds of scheduling algorithms and their pros and cons.
I don't think that it's backwards compared to other schedulers (many of them are monolithic) but I think it can be improved by using transactions and optimistic concurrency control. This is discussed in the Omega paper and I think there's work to integrate the shared state scheduler into Mesos.
The middle lane is not shared, it will only be used by one direction at a time and a middle barrier is usually separating these two. Wikipedia has an article explaining it further: http://en.wikipedia.org/wiki/2%2B1_road