I was just doing a personal survey on this problem today and I think may be is a bigger issue on Parenting than on any other SE site I've explored so far. I do think it could kill the site, making it insufficiently different from any other online forum. There will be some people who are just disrespectful preachy jerks, but I suspect that most people would actually try to make their replies better quality if they were just reminded of the principles more often. Many people probably read the FAQ once, and then forget where they are. Plus it improves the world generally if we can train people what good and bad answers are.
It's too laborious for questioners to have to "defend" their question and to lecture respondents about the difference between an answer and an ever-widening discussion or an (unwelcome) refutation of the premise that the question is based on. If a new user of the site has their factual question just sucked into a tiresome debate, they'll probably never return again. The same is true if they feel their intelligence is being disrespected by people making tiresomely obvious critiques of their question (like, "the brand of diaper you use will not ruin your child's life... life is more complex than that, so don't worry about it!") When I respond with an answer that is weak or possibly too obvious, I at least try to confess that I've done so, so the questioner does not think I'm disrespecting their question or their intelligence.
A questioner is under some pressure to be a gracious host (not wanting to scare away additional answers) and so may not feel able to scold a respondent who didn't really ever get around to answer the question, but who at least made a well-intended effort and (finally.. what a relief!) didn't drag the question into the weeds or provoke an argument. Therefore it will be critical for the Community at large to diligently point out non-answers wherever they occur, and not leave it to the overwhelmed questioner.
Some technical features might help:
-- More automated FAQ mini-reminders that actually block the user until they read it. E.g. I've noticed there is now a pop-up when you start to type an answer (reminding you of good practice) which is excellent... but it's too subtle and easy to proceed without actually reading it. Also the popup is only seen by people trying to post answers... unfortunately many people who have the impulse to argue or preach like to do it in the comments, where there is no such reminder. Reminders might bring users to a full stop when they first have enough rep. points for some privilege... and could become less obstructive as more points are earned.
-- A handy list or palette of "deep links" to specific items in the FAQ (including SE Meta Community FAQ), so people can quickly link to a reminder of proper behavior without having to explain it from scratch.
-- Perhaps Wikipedia-like 'templates' could be used to easily flag specific common problems with a question and automatically link to a FAQ, giving the poster a brief opportunity to fix it themselves before being downvoted or scolded more thoroughly.
e.g. for Answers:
{{fails to answer the question}}
{{misreads the question}}
{{opinion doesn't cite source or personal experience}}
{{needs reformat to lead with answer}}
{{unjustifiably assumes the asker is simpleminded}}
{{preachy}}
...etc... (though these long names may be too cumbersome)
for Questions:
{{question unclear}}
{{question too broad}}
{{question too narrow for anyone else to care about}}
{{controversial topic please respond carefully}}
...etc.
for Comments:
{{should be an answer}}
{{should be a separate question}}
{{unjustifiably assumes the asker didn't intend to ask precisely what they asked}}
{{inappropriately provocative}}
{{playing the censorship martyr card}}
...etc.
Maybe someone could work out a list of common faults that templated responses might work for.
Any user could add templates as comments, and moderators could edit-add them inline.
The final result may be the same: moderators sweep it all up afterward. But this could save a lot of tedious explaining when people commit common offences. And the fact that something is a template helps to point out to people just how tiresomely Common a fault is.