Would you trust some random island state with your multi billion wealth? Seems risky. Also how do you maintain access to all the relevant markets that you need to multiply your wealth?
Well, you don't have to actually store your wealth where the island state can get to it. Just because you're resident in the Cayman Islands for tax purposes, doesn't mean you need to hold your wealth as Cayman Islands Dollars, or store it in a Cayman Islands bank.
Banks in Switzerland and London are more than happy to look after your money for you, denominated in whatever currency you choose.