I have no idea how it relates to the later guides as I'm in Preparing with my oldest now.
But I can tell you that going through things in Preparing is totally different than other Bible history.
My kids are well entrenched with Bible history. Our church is awesome about it... they go through from creation to revelation every three years, meaning it takes them three years to get through that much information. They do this in Sunday School and also in AWANA. So twice a week they are hearing about Bible history and repeating the cycle every three years.
Plus as a family we talk about the Bible and read through the Bible. Jayden is also in a kind of co-op class that is creation to the Greeks? Creation to something... I can't remember. Basically the first half of Preparing, but taught in a different way with different materials.
Jayden has learned SO MUCH through Preparing. Honestly *I'VE* learned so much through Preparing! But then again I was in public school and I never really put it together that the Bible and everything else in history were happening at the same time
Anyway, I would definitely not skip anything or think it's redundant. It puts a completely new "spin" on everything.
Plus, I don't know that you could ever go wrong with more Bible learning.