I see two issues at stake here. One is snark. The other is preachiness. Conflating the two is going to cause problems with the argument. Examples would be nice as this is such a case-by-case topic that there will never be a one-size-fits-all solution. One person's / culture's snark is another person's / culture's legitimate teaching tool.

There has been a noticeable increase in answers that begin with snarky/preachy statements followed by an answer

Let's not conflate snark and sermons. The sermon is often the true answer. The "answer" that follows the sermon is often instruction on how to explosively remove one's foot. So I would argue to not to discourage the sermons. If someone wants to have instruction on how to make an open relay, let's all agree that a sermon is in order. Conceivably, it's possible that an open relay internal to an organization would solve a very speciated problem and so be a legitimate question that requires an answer. However the sermon would be good for any future readers who do not have a legitimate reason to do that.

As an anecdote: I've learned much from forum posts and magazine articles that didn't answer the direct question, but instead preached against the original idea. I just didn't know that what I wanted to do was a bad idea. (Thoughts of trying to finagle a Windows SMB share to mount as a drive letter a few years back come to mind)

But I don't want to pull things off topic as I think the problem you're mostly addressing is snark and rudeness.

At least one user has been temporarily suspended for being rude.

My first though when reading this sentence was "Rude and snarky are not quite the same thing" And then I saw the next sentence...

Being snarky, is in reality, no different from being rude.

I am not yet prepared to agree with this statement. Perhaps it's a misunderstanding of terms though.

Everyone - please refrain from snarky answers.

We will first have to define the terms. What exactly is a snarky answer? As it's subjective, I think people will always have disagreements on this topic until the sun goes nova. Another anecdote: I'm an American. I've worked with Australians for a few years now. Our two cultures have caused some mild clashes, misunderstandings and dinged feelings. There have been some cases where I thought to myself "I guess that person just doesn't like me," when in reality there was no such dynamic. I later discovered that what I thought was rude was in reality not rude by another culture. In fact, I was actually liked.

Rude may not be rude. Snark may not be snark. We must define terms as concretely as possible but leave wiggle room for a variety of communication styles. Certainly questioning the virtue of a person's sister is always going to be rude in any culture. Saying "are you kidding me?" to a proposed design isn't, however, black and white. Some places would call you into HR for that. Others wouldn't even know that a trip to HR could even be considered in that scenario.

I've seen posts that I thought were rude, but they might not have been. There are posts that others have written which I've read that some people would take offense to, but I am completely oblivious to any rude undertones.

#Summary

On the topic of preaching. I cannot get behind any effort to keep the true motives of a poster from being found out and the true solution to the true problem from being given. Question A does not always need Answer A. It might need Answer 14c § 9. But to reiterate, I don't think this topic is at all about "preaching" as it is about rudeness.

On the topic of snark and rudeness. I do support the increase of politeness and hope we all do too. However, the perception of snarkiness is what's always going to cause trouble. I think we all know that "a certain someone who was recently banned who shall remain nameless" crossed lines because his snark was most certainly ad hominem. Personal attacks should never be okay in a public place for discussion. Certainly, there are some users who are more sarcastic than others (I'm thinking of two non-ban-hammered people in particular). Can it be called outright rudeness? Only that person can say for sure since only they know the intent to their words.

For myself, I'll try to be more careful with possibly ambiguous statements and be more cheerful and openly polite. Perhaps that's the only option that will ever be acceptable in a public forum with so many different people and cultures.