ETF might be useful for those ordinary people that have not enough knowledge to store their bitcoins in a safe wallet. But generally speaking, I don't think ETF is helpful for cryptocurrencies world. And the reason is simple. ETF acts as a third party and we use bitcoin and other cryptocurrencies to eliminate third parties.
Some people might think that as ETF provide an easy method to buy bitcoin, it's useful. But it's wrong. These people are those who have not enough knowledge and it's better to be out of the market. Bitcoin market doesn't need them as they help third parties get involved in our transactions.