People scream ACiD on NoSQL's because they fail the easier A, C, or D in various forms, not the harder I part. Having AC&D go a long way in ensuring data correctness.
Unlike "C" and "A" in "CAP," "AC&D" (specifically, "C") can't be easily separated from "I".
Serializability ("I") ensures that database consistency, or maintenance of integrity constraints ("C"), is not violated. While it's possible to get consistency ("C") without serializability (which would give up traditional "I" in favor of a weaker form of isolation), it's often difficult [see http://www.bailis.org/blog/when-is-acid-acid-rarely/#arbitra...].
Serializability ("I") ensures that database consistency, or maintenance of integrity constraints ("C"), is not violated. While it's possible to get consistency ("C") without serializability (which would give up traditional "I" in favor of a weaker form of isolation), it's often difficult [see http://www.bailis.org/blog/when-is-acid-acid-rarely/#arbitra...].