Not sure if you meant to imply that they do.

No, I don't imply any connection to anencephaly. The point is just that many deleterious outcomes that we observe are a trade-off against something. In some cases we know against what -- as you mentioned, sickle-cell anemia is an example. But in most cases we do not know. I would expect that a prior of "it's a trade-off against some advantage, we don't know yet which one" to hold as a rule, but I also expect to find some exceptions to it as well.