I couldn't care less about multiplayer, I'm in it for the campaign. And if the campaign is normal length (8-12 hours long) I'm buying it day one. If it's 3-6 hours long, I'm renting it or buying used.
Even if we assume that the 6 hour and 12 hour games are both good quality for those lengths of time, there's such a huge variance in what we would call "good quality" that there's no way to nail down exactly how good a 6 hour game has to be to have more value than a 12 hour game. The time vs quality metric becomes so subjective that it's completely useless to even discuss it with other people. A reviewer might include it in a review, but all he can do in that case is give a personal impression that may or may not be of any use to the reader. How could he justify that impression rationally in a way that we can understand and relate to?
Point being, it's a silly thing to worry about imo.
Your point on campaign quality makes sense. A 3-6 hour campaign may work for the right kind of puzzler or something, but I don't think there's a campaign good enough to justify that short a length for an fps. Especially without an extremely deep multiplayer mode or at least great graphics as eye candy.
I'm hoping for the best, but I'll keep my expectations low at the same time. So far this game looks and sounds very uninspired. Either that or extremely rushed.

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