Who you calling "Scabs!!!?" Oh, right.
Yes, each addition to the game has its merits. No, they don't form a coherent path to Necro gaming glory. They repeat rules. They update rules. They omit rules. They don't build on each other. To proceed from HW to full campaigning, you still need two rulebooks, or three, or only one, or two more, or another one from another boxset. It's certainly worth the journey, but not for the faint of heart.
In comparison, my longtime Necro group just took up Blood Bowl for a diversion. A Fall dalliance, if you will. Guess what? One thoughtful BB boxset does it all. One rulebook to rule them all. Unlike Necro, four of us bought the new box, and now have each have a BRB and game board and teams. Ready in case a Blood Bowl game breaks out (hope springs eternal). Now, I'm talking two guys who NEVER buy anything. And, yes, just like Necro, the one of us who refuses to buy any rulebooks because online rules are free and "easy to use," still invariably ends up with one (or more) of our hardbacks in his possession for most of the game. Grrr! Oh well, at least he's cute and cuddly. There is one expansion for those so inclined. A reasonable amount of expansion accessories. It's an imperfect comparison, but an illuminating one. The BB boxset is a one-and-done, one. HW is the first step out the door on the journey.