In linguistics, there are two camps known as the Prescriptivists and the Descriptivists.
The Prescritpivists believe in the one right way of grammar. Some form are Wrong because the Books Say So and the Law Must Be Obeyed. People who disobey the rules should be locked up by the grammar police.
The Descriptivists, on the other hand, talk about how language is used. There's no right or wrong, as long as the message is communicated.
As you can imagine, neither approach works on its own. Real language is a combination of the two: http://jacobian.org/writing/descriptivists-and-prescriptivists/
Etiquette is similar, I think.
That's a really good way of explaining, and I agree with you on how etiquette works.
I end up saying this in a lot of threads, but I think etiquette has two components: the Rules, and how you apply them. The rules need to be rigid to be useful, but their application needs to be more fluid in order to respond to individual circumstances and situations.
If you're too rigid, you mostly just come off as an unfeeling, disconnected snob. But if you're not rigid enough, you may as well give up on the idea that you can have a set of rules at all. When every rule has bunches of exceptions, what purpose does it really serve?
At least for me, it helps me to determine when to be rigid and when to be more loose if I divide it into rules vs. application of rules. Rigid rules and loose application gives me room to be understanding and realistic without losing all consistency.