Considering this is often a game of courtly intrigue between noble houses, it's probably more important than most would assume at first. But this skill doesn't just mean upwardly mobile social skills- it's a reflection of knowing the proper mode of behavior for any social group, so a noble with etiquette 5 would probably grasp how not to talk down to an angry mob of starving peasants and avoid being strung up.
0: So, the screaming mob wants you to scream along with them about stringing that person up, right?
1: Oh, they really, really, really did.
2: But you'd never offend them like the prodigal would by shamanistic greetings to violent Oathlands Orthodox fanatics.
3: Who prefer a greeting that's at least from the Faith of the Pantheon in their specific style.
4: For this particular sub-sect that's been condemned for heresy that you carefully don't mention.
5. And have secret rituals you're passingly familiar with.
6. Because you know how they were created and what they originated from.