Except that I have seen way too many incompetent uses of APIs that support bad ideas, and so, in reality, everyone would use "the best setting!", aka, all validations switched on, and thus probably lead to all kinds of false positives with hard rejects. See also the common rejection of +-suffix addresses, for example.
For address validation, connection checks are a bad idea. For address validation, turning any temporary failure into something that could end up as a hard reject is a bad idea.