Hmm, the playtest instructions do actually say, "Rights give individual PCs their own unusual abilities, modifiers to the moves, or other situational beneifts. They can have both game-mechanical and purely fictional effects."
I think it's clear that taking some "rights" on your character sheet actually confers abilities, just as that line states. They aren't wholly about social contract or perception of others. If you take the right to correspondence in Greek, it's a reasonably safe bet that you can actually read and write in Persian... and if neither Persian nor literacy is particularly common in your setting (and by default, in a Dark Age of a Northern/Western Europe different from our own, they wouldn't be), then most other people simply can't carry on such correspondence. There might be some who have the ability but not the right -- slaves who were formerly scholars in the East and are now forbidden from writing, say. But there are others who have neither the right nor the ability. The right, in addition to being a right, has also "granted you an unusual ability" which others do not have. It's not that everyone can read Persian, but only you will avoid getting in trouble for it. It's that they actually can't read Persian.
There's something about "anyone can do any enchantment" that is attractive if you think of enchantments as having to do largely with summoning the old gods, folkways that are available to those willing to take the risks involved. But there's also something about it that bothers me a lot, because the Dark Ages were also full of mystical, hermetical, and other secret magical traditions, and even in sort of hedge-witch paganism there was clearly the sense that some people were special and knew special secrets. The idea that the only thing keeping the War Captain with a -1 Weird from sacrificing seven people and permanently turning a dozen of his warband into unkillable zombies, and the only thing stopping him from doing so is social sanction (including the sanction of the gods)... that seems to pretty radically decouple magic from "unusual abilities" and knowledge in a way that seems problematic...