I like two things about the book:
1) It's just enough information that someone who doesn't want to build their own setting (or wants a launching point for doing so) can actually make use of it, even if they don't want to go all-in on something with more historical information than some real-world civilizations.
2) It bridges the gap between throwing out existing books (by virtue of replacing them) and leaving the setting without an anchor for the current edition.
I have a 25 year, standing ban on buying anything related to the Realms. It started out because I found the setting lackluster, if appropriately detailed, and morphed into disdain of specific elements (key NPCs) and the overwhelming amount of detail from stacks of game source-books and poorly written (IMO) novels. I'm very impressed that 5E continually challenges my resolve on that ban. Right now, it remains in place because I want to send a message to WotC to support other settings. If they telegraph a commitment of official support for another setting, I'll relent and pick up SCAG.