Is there ever a reason why I would need multiple core sets?

except using it for custom made stuff or because your game is broken for any reason, no, there is not.

Not really. The only thing I can think of is actually being able to play multiple groups of the same creature, but that's a small (and likely confusing) benefit as a pretty high cost.

Okay, good. Because I seem to remember, when I was just looking through RTL to see what it was like, multiple Goblin Archers spawning, and I wondered why it was doing that.

Well, it's a small thing, but I'd just die to be able to fill an entire encounter with nothing but goblin archers.

Fortunately for all the heroes we'd like to overrun that way, there's the rule that all reinforcements follow group limit unless explicitly told to ignore them. The group limits, in turn, are set by the supply- which does not grow with multiple copies of the game (as only one of each set can be included in any given campaign). In other words, you can own as many goblin archers as you want. The rules still say:

1) You can't assign 2 groups in an encounter as the same monster group (not even if you've got a monster duplicated by the CK and a hero and monster pack)

2) The group limit of goblin archers (4 heroes) is 4 minions, 1 master. Not 4/1 per core set.

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1) You can't assign 2 groups in an encounter as the same monster group (not even if you've got a monster duplicated by the CK and a hero and monster pack)

[...]

Where does it say that? I remember looking for it, way back, but I never found anything on it, and since then I've just been assuming that if you have two different groups of the same monster, those groups count as separate groups. I've also heard it mentioned several times (not just here) when the question of "multiple copies of <product>" comes up, so I'm not alone in this assumption.

Not questioning the claim or being contrarian, I'm genuinely curious. Such a rule would make sense to me, which is why I thought it funny - if not odd - when I couldn't find such a restriction. The idea of multiple groups from <set> is usually just an interesting tidbit or general musing, and it's perfectly understandable (to me) that there wouldn't be an explicit rule against it, since it's likely extremely uncommon and unexpected that people would have multiple sets of the core set, or an expansion.

But if there is, I'd like to know where.

Edit: On a separate note, I've always found the rule of "limited by the supply" to be annoying precisely for reasons such as this. By that logic, you should absolutely be able to expand upon the supply, but this would most definitely cause issues in some cases, which the supply is never considered to become bigger - but then, why limit it by supply, and not just state a number? Infuriating wording with little to no practical application.

Regarding the "limited by the supply", you've got to remember that "the supply" gets defined at the beginning of a campaign (if playing that way) or at the beginning of a session (if standalone). This occurs during step 4 of the general setup if you want to get really specific.

In each expansion, there are instructions ("Expansion Setup") which basically says that if you're going to be using this particular expansion, add the included components to the supply. That is, you choose exactly which expansions will be included in a given session/campaign, and that determines the supply- not what you have lying around.

When choosing monster groups (see the inside cover of the Shadow rune quest guide, this may have been updated to the inside cover of Heirs of Blood in the new sets) it specifies that for open groups you must choose an unused monster group matching at least one of the available traits. Therefore, no duplicates.

Regarding the H&M/CK overlap, in each hero and monster pack there are instructions to replace any CK monster groups (or heroes) with the version found in the pack.

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When choosing monster groups (see the inside cover of the Shadow rune quest guide, this may have been updated to the inside cover of Heirs of Blood in the new sets) it specifies that for open groups you must choose an unused monster group matching at least one of the available traits. Therefore, no duplicates.

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There it is.

That being said, I think there's an argument to be made that that refers to a monster group in itself, meaning that two different monster groups of the same kind could still be used.

As in, I have two monster groups consisting of Goblin Archers, and even if I deploy one, the other one remains unused, and can thus be used.

I do not think that is the intent, however.

You could make that argument. I would then have to remind you that technically correct, while the best kind of correct, is not always the most correct. I would then have to admit to being rather hypocritical for discounting the value of an argument based on phrasing.

Let's just simplify the situation and agree that if goblin archers are already in the quest, "goblin archers" are used.