Need to withdraw diminishes as the network expands; the IOU becomes a functional placeholder for the currency. Depending on the currency, withdrawal from the gateway could be rapid---and potentially instant if banks begin to act as gateway services.
No they don't. We have already determined that you won't get IOUs from anyone you do not extend a trust line to. Therefore the IOUs are not generally tradeable.
If you guys don't understand the system then ...
They are generally tradeable, that's why market makers exist in Ripple. As long as there is a market path between 2 currencies, you can make a transfer in Ripple. You don't need to trust the issuer that the recipient trusts.