That some people use email addresses that are personally identifiable in the non-visible portion of their profile but have an otherwise anonymous profile whose comments they do not wish to be traced back to the maker. For instance an Apple employee that speaks about Apple internals anonymously might get sacked if they were exposed.
Right, and just so it's 100% clear to everybody: we never did and never would knowingly publish this data. A small amount of it leaked (for 30 seconds on 3 occasions) because of an obscure mistake in our code, and we're deeply sorry about that. We turned off API publishing the instant we found out about it and dropped everything until we were sure it was fixed.
The API design has always been to publish only information that is already public, that anyone could get by scraping the website.