I often find myself trying to explain why linguistic fine points -- like using "they" as a singular pronoun -- matter. I don't believe it's just a question of snob appeal. The latest explanatory term I've latched onto is "cultural signifier." It's a bit too ivory-tower for my taste, but I don't know a better one. The idea is that the linguistic fine points we pay attention to -- the accent we have, the vocabulary we use, the knowledge of traditional grammar we exhibit -- say something about us. They hint at both the culture we come from and our place in that culture, and there's no getting away from this.
Fine points of behavior on the street are a cultural signifier too. As Fallows says, they tell you something about a culture and about individuals within that culture. What I like about the analogy with language here is that no one seems to need to have it explained why fine points of street behavior matter. We all notice it, we all judge it, and nobody seems to think it's snobby or pointy-headed to do so.
Maybe instead of invoking "cultural signifiers," I'll start explaining the point of linguistic niceties like this: "You know how you feel when you're out walking and a group of people coming toward you takes up the whole sidewalk?"