Agreed. It just happens to make up for a lot of things that gimp the Sisters. Of course it would be nice if we didn't have to use it to make up for stuff, or if there were other nice options too, or if the points were better balanced and we didn't have to pay 15 points for a Meltagun on some squads, but yeah, there you go. Would it be overpowered in another army list? Hell yeah, but the Witchhunters list just isn't very strong on its own.
And really I completely agree with your analogy to ATSKNF. If you took the same WH list and just took out the BoSL, it would be like taking a Space Marine list and taking out ATSKNF. The book is just that important. It is a piece of wargear, and maybe wargear should simply add useful options, but that's just not how it turned out. Certainly if someone took the WH list and removed only the book, the WH army would be much weaker, about as if someone took ATSKNF from the Space Marines and effectively make the army even less competitive than it already is. I mean it's not like the Book is making unstoppable cheese build Sisters armies here. We use the book and it just happens to help us actually win once in a while. It's not like they're Long Fangs.