Denigration, to borrow your term, occurs only when someone insists that--in the real world--there are no standards and all usages have equal merit. There may be no rational reason why one usage wins out over the others, but that's the way it is.
I agree with your last sentence. Certain kinds of English have prestige. And sometimes the reasons they have prestige are arbitrary. We have what we can call a standard English, as defined by usage guides and dictionaries. And usages that differ from the standard, like say for instance, "irregardless" and "I could care less", are condemned. That is what I mean by denigration.
Since I have a background in linguistics, I'm interested in looking at how language is used, and not only in how some people say it should be used. This includes looking at nonstandard usages. Some of these might be fads, as you say, and some might provoke a more widespread change. But they're all interesting. And since this is a language forum, they are worthy topics for discussion.
Great works of art and writing are achieved by those who break established rules. But they break rules with style and skill, and create something of lasting value in the process. Merely ignoring standards willy-nilly isn't the same and doesn't deserve the same respect.
Speakers of all languages, whether they speak Canadian English, African American Vernacular English, Texan English, Scots English, Hixkaryana, Japanese, Quechua, whatever, are all following rules. The rules might not be written down in a book, but the rules still exist. They exist as as unconscious knowledge in the minds of the speakers.
Whether or not the speakers are following or should be following certain standards is a separate issue, and not one I'm talking about (although admittedly I might not have made that clear).
For instance, why do we say
An old stone wall
*A stone old wall
You won't find this rule in any usage book. But the rule exists.
So what if we look at "irregardless" and "I could care less"? They seem to have too much or too little negation, and yet they still make sense. The other examples I gave show the same phenomenon.
So in short, I don't completely disagree with you, but it seems like I'm approaching the topic from a completely different direction from the rest of you guys.