I think learning something
just because other people know it is sometimes a good reason. Yes, it may be arbitrary and a little silly that most people have a passing familiarity with Psalm 23 and not Psalm 22 (which I happen to think is a much cooler psalm): but if you don't have the time or inclination to teach your kids every psalm in the book/chapter in the Bible/religious text in the world, why not go with the one that is used/referenced most often in society, to save your kids the embarrassment of not knowing something most other people know? Or, to choose a secular example: I don't have the time or inclination to teach DD how to say a few words in every language under the sun, but I'll teach her to say a few in Maori. That doesn't mean I think Maori is a "better" language than, say, Swahili, but it's more relevant to our lives and frankly, people will think she's somewhat dim if she can't respond approprately to "Kia ora" or "Haere mai".
I think Roman numerals come under the category of Stuff People Know, and it'd be doing your kids a disservice not to teach them without a good reason. They do pop up here and there - plus, they're fun for developing logic and math skills, and are a good precursor to algebra (showing if a number can be written as V instead of 5, it can also be represented by another symbol such as an X).
In other words, I think the "all random facts are created equal" line is specious. Most people don't know Greek, so your child is unlikely to be disadvantaged by sharing that ignorance. Most people do know how to use Roman numerals, so your child may be disadvantaged by not knowing them.
Plus, they're fun and easy. I taught a five-year-old with autism how to use them and he thought they were the coolest thing ever.
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