So in the end again you don't trust your bank, but the FDIC (in the US for example) that they have always enough funds available at all times to guarantee you at least 250000$ in your account. I'm using absolutes here, because any even tiny doubt would reduce the worth of the IOU from face value to below it.

Any kind of more beneficial terms (e.g. higher interest) then makes the IOUs more attractive - but that just means you are willing to take on more risk for a potentially more rewarding IOU. This just offsets the loss in value because of less trust to bring it up to face value because of more rewards. Why shouldn't this be possible in Ripple too?