You should think about the ramifications of your statement, because everyone has a need to keep assets in IOU form on a daily basis....they just don't recognize it. If you want your money to work for you, keeping it liquid is the worst advice you can ever follow, as inflation will eat it. The second you put it to work, you're dealing with IOUs.
You are talking about Fiat money. Fiat is IUO if it is not in form of cash, and even cash is an IUO to the FED (originally it was the promis to be convertable to gold...).
For BTC that does not apply. The money system and the credit system should be seperated. Fiat comes as a merge of them. Ripple has the same nature. BTC is pure money, you cannot lend your BTC directly via the BTC system. I am not against lending. People should be able to do that business, but that should be a seperate business, like insurances, hedgings, bettings, whatever....