There are numerous threads where I've talked about my family situation, or my MIL, or my son. Those are comments buried in what must be millions of threads, so I don't worry overmuch. If it's in the DM, I may well be identifiable, and my MIL will almost certainly read it. She would then have my user ID and can search MN to her heart's content. That's a can of worms I'd really rather avoid. She's my son's grandmother and that matters more than a website.
The MN books talk about issues specific to various child rearing stages. They aren't personally identifiable, and they aren't read by a mass of the population. This is, IMO, very, very different. I'm not saying it's wrong - it's for each individual to determine what they're comfortable with, obviously, and probably many people will either not care or be positively flattered - but I personally will not post about my MIL, or a dispute with my mother or husband, or info a SN sibling when just trying to help someone understand that "high functioning" doesn't mean "misdiagnosed", if it's going to be served up with a twist in a national newspaper. Frankly, I value my real life far too highly. It's my loss, not MN's, but it makes me pretty sad. I like this place.