What matters is not whether they do; what matters is whether the overlap is 1) sufficient to allow skill checks in two skills to count when answering the one question; and 2) important enough to be taken into account by DMs when adjudicating such matters.
I don't really see the distinction. But, phrase the question any way you want to, are you claiming that no question that could be answered with a know: religion check could ever also be answered with a know: the planes check?
Uh-huh. I hate it when I overcomplict something. It does terrible things to my compliction.
All I am saying is that I am implementing rules directly out of the book and you are jumping to things like
"That way lies madness.
What next, skill defaults?
Optional specialisation?
Maneuvers?"
It is not remotely that complicated.
If I want fine detail in my skill lists, I know where to get it. D&D's strength is not in faithful representation of reality, it's in fast play.
And you can have a fine list. But Monte should not be shackled by your play style.
If you are going to impose a fixed realm of knowledge skill on your game, then you will be forced to adapt when you use new material. No big deal.
But to argue that it is a bad thing, just because you don't want to play that way does not seem reasonable.
Again, if you are claiming that anything I said is an attempt to create a "faithful representation of reality", you are really overcomplicating it.
Fast play?
Player: Do I know X?
DM: What Knowledge skill do you think you would use?
Player: Know: XYZ
DM: OK, the DC is A.
Very quick.
Here is how I imagine your version:
Player: Do I know X?
DM: Sure, make a Knowledge: Religion check.
Player: Religion? I don't have religion, shouldn't I be able to make a know: the planes check? I have 14 ranks.
DM: Nope, this is a strictly religion matter, the planes have nothing to do with it.
Player: What do you mean, this question is all about the planes.
DM: But it is more about religion.
Player: I disagree.
DM: I'm the DM.
Player: nevermind.
... except hunter of the dead, which is what this thread is about in the first place. Note that even the undead-related PrCs in T&B (pale master and true necromancer) don't have a Knowledge (undead) requirement. It's a one-off thing that Monte came up with, and with any luck, it'll die out.
Like I said, if you are going to constrain your game, you are going to have to adapt material to fit you view.
Change the prereq to know:religion and you are good to go.
I think the addition of knowledge skills adds to the flavor in the game. With any luck, it will catch on even more.