I see the logic, but I don't think it's a great solution.
I think the logic is that the Knight and Slayer (for example) are unique from other Fighter builds and from one another. The pre-Essentials Fighter builds are more similar to one another than to the Essentials Fighters. Therefore there are three types of Fighters -- pre-Essentials Fighters, Knights, and Slayers.
Personally, I'm not a fan of conflating build with class. I think Knight and Slayer character entries in the CB should list their class as Fighter, rather than my warforged battlerager's class being listed as Weaponmaster -- a designation that doesn't suit him at all.