Because when an exchange becomes a ripple gateway, you can withdraw fiat IOUs and trade them for another exchange's fiat IOUs. Which you can then redeem (deposit) at the other exchange. Presto, fiat moved without a bank transfer.
You're going to find that GoxUSD IOUs are worth less than StampUSD IOUs, which won't solve the problem then.
GoxUSD might be worth less, but we would find it to be a lot closer to face-value than the current discount rate (there would be much more arbitrage of goxUSD if it could be withdrawn to ripple).
Also, by enabling the trading of fiat IOUs with face-value, the market could directly discount the fiat IOUs (by the spread in bitstampUSD->XRP->mtgoxUSD). Then BTC prices should be more-or-less the same, which would be much better than a massive spread priced into BTC at every separate exchange.