You don't get frost giants if you don't get Monster Manual N? What? They think, people are going to be more likely to buy Monster Manuals II and III and so on if they don't get all the major iconic monsters in the first one? That this won't just knock those monsters off their iconic pedestal?
I assume they know what they're doing, but . . . are most people really going to get more than one monster book? How much is Wizards declaring "these are all core books!" really going to affect how people use these books, and the monsters in them?
I do see how this could work. Package the monsters in each book so that each one gives you a particular kind of campaign. But just spreading the classics around, which is what it sounds like . . . I just don't know how well that'll work.