I think evaluating the BP on its own is perfectly valid. I mean, they do represent a commitment to invest around £9 a month which is not, I grant you, a lot of money, but it is money. Taking the cards from one BP and looking at them and thinking "I'm not terribly impressed with these" is thus a valid decision. "Skavenblight Threat" is a product on its own - they didn't produce it in a box like the core set with the other sets of its cycle, and thus it can be judged, both as a product individually and as the first release for its series of cards.
Granted, it's equally valid to delay judgement on the first booster until you have a wider idea of what the metagame holds, but nonetheless a pitfall the expansion must strive to avoid is a failure to generate a decent level of excitement with the cards contained in the pack. Also, they should probably measure them before they go out. Just saying.
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