Useful? It's gambling, gambling on whatever system is backing the currency, like the bitcoim blockchain or usefulness of anonymity by monero, USD pegged currencies beinf used to avoid needing dollars for international trade,etc.... or maybe people just like to gamble sometimes. No one accused blackjack of being useful to society.

I would argue that blackjack shouldn't be part of our financial system to the point that it needs regulation.

But if people are playing blackjack with their life savings and it has widespread use, why not? Crypto is speculative trading.

I seem to remember a "why not" for exactly that scenario sometime around a hundred years ago...