So let me get this straight any gateway that allowed IOUs from other gateways would be taking on credit risk and in the event of a run would not be able to pay back 100% of it's own IUOs because of the IOUs of other gateways it held were defaulted.
You have said that it's people own fault for using these kind of gateways that are exposed to the above credit risk. So you would therefore say that all people should use gateways that only redeem IOUs that they issued themselves (and are therefore able to pay off 100% of their IOUs)
So lets assume all gateways only redeem their IOUs (because people would be silly to use a gateway exposed to risk you said).
Person A logs into Bitstamp, deposits Bitcoins, sends them to Ripple a Bitcoin IOU
Person B logs into Weexchange, deposits USD, sends them to Ripple a USD IOU
Person A trades their Bitstamp Bitcoin IOU for Person B's Weexchange USD IOU.
Person A logs into their own Weexchange account and redeems the USD IOU and withdraws their USD
Person B logs into their Bistamp account and redeems the Bitcoin IOU and widthdraws their Bitcoins.
Now ask yourself, what was the point in all that? When they could have both just logged into Bitstamp and done the trade directly without ever having to go to Ripple?
The key here is that Ripple creates economic incentives for people to mediate this kind of transaction. Person C can hold WeExchange and Bitstamp IOUs, and let Ripple trade them for a small fee. Person A would only use WeExchange, Person B would only use Bitstamp. If Person A wanted to send USD to Person B, Ripple would give Person C their WeExchange IOUs and give Person C's Bitstamp IOUs to Person B. There's a market for Person C's fee, so it will tend to approximate the actual cost of moving money between gateways (in bulk, after netting.)
If nobody's providing this service between two gateways, there's a huge incentive to step in and make it possible, because you can set whatever fees you want. If the fees are too high, there's an incentive to step in and compete by trading IOUs with lower fees. It's more or less inevitable that if there are two gateways in use for a certain currency, a market for liquidity between their IOUs will spring up. Currently I understand these markets are rather sparse and illiquid, in part because the client doesn't have support for setting those fees yet, and in part because there's only one major gateway right now.