Right. But if you know a good chunk of it is nonsense why would you assume the rest of it is good? A lot of it is definitely bad, and the only way you're going to know if a part of it good is if it compares favorably to stuff you already know is good.

It's useless as a primary source for historical info because there's no way of distinguishing the crap from the rest of it without using verified sources.

In the case of the science book, you can probably test out a few things to see if they generally hold up. Gravity info in a science book is probably still fine even if they botched some of the details on friction, or something like that. Unlike history, you can verify a lot of "science" without comparing it to other books, because science ideas tend to be testable. They're very different "sources".