We need a functioning Carbon trading/tax regime. I don't care if does something useless with a computer, that's literally most people, I care that we don't make people pay the price of their insanity.
Especially when you have e.g. the aluminium smelting industry that uses around 10x more energy*. Arguably more useful, but the externalities of using unclean energy for that process, and all other unclean energy consuming industries, should still be priced in.
If cryptocurrency miners want to use that energy, they can knock themselves out, but they should at least be strongly economically incentivised to do it in a CO2-minimizing way.
* (15,000 kWh per tonne) * (63.2 million tonnes annually) = around 948 TWh sources: - https://aluminiumleader.com/production/how_aluminium_is_prod... - https://agmetalminer.com/2015/11/24/power-costs-the-producti...
Some (mostly already very wealthy) people want them. That is not the same as a genuine need.