I tend to think of Etiquette as an academic field not typically recognized by the academe.
Basically, it has a lot in common with, say, history or sociology, or even the hard sciences (as understood by professional academics, anyway). It's not so much that there are facts or anything like the Truth, nor is there anyone engaged in studying or writing about it who is infallible.
Instead, Etiquette (like most academic fields) has a body of writings and knowledge that come together to form the discipline. Collectively, they define what is considered correct and what is considered not correct. Certain individuals are more influential than others, generally because they have contributed something to the body of knowledge that most people find to be correct. But even they are not "right" 100% of the time, and in time, other influential individuals will replace them.
So it's more like a theory than a fact. It's a perpetual work in progress, always being refined to respond to and incorporate new evidence.
Given my viewpoint, I consider Miss Manners to be an etiquette expert. I give her interpretation of an etiquette rule the same weight I would give a history professor's interpretation of a historical event. That is, I think it has more weight than someone who has not made a career out of studying the topic, but that does not make it inherently correct. If someone else can present a convincing argument, it is possible I will be swayed. The trick is that Miss Manners herself (and other etiquette experts) present pretty darned convincing arguments, so it takes a lot to be more convincing than that!
Aside from that, people who set out to write about etiquette in the aggregate are less likely to be responding to circumstances that are mostly regional or confined to a series of anecdotes. Most of us who do not set out to do this as a career are limited in our observations of how the world works, so it's much more difficult to make generalized statements about etiquette as a whole. I know what my friends and family do, but that doesn't mean that what they do is the most correct, you know? The best arguments that I (as an amateur etiquetteist) can make are the ones based in logic, rather than in observation of social norms.