Singapore is a city state. I don't think they want to be involved in letting other countries use their currencies! Really, the economy you borrow your currency from has to be huge to support that, so there are really only a few choices: USA, Europe, maybe Thailand could (but it is still quite small), maybe Australia, and of course...China (but RMB doesn't float, so a non starter). Dollars have stability that is on par with gold (or even better, recently), so why not just go with that?