Because using BTC.Bitstamp on Ripple you can trade over a dozen different currencies both fiat (e.g. CNY) and crypto (e.gg LTC). Using BTC.Bitstamp on Bitstamp internal, you can buy XRP at inflated prices or trade USD.Bitstamp, that's it.
Please send LTC using BTC (it would even theoretically possible!)...
With Ripple you can choose which part of the banking system (if at all) and which interface you want to use. You pay fees only for clearing balances - if you needed to pay banking transaction fees (or Bitcoin transaction fees for that matter) for every single trade on Bitstamp or MtGox, you'd not trade much any more.
I can move LTC to BTC just fine using any existing exchange, such as LTC to Btc-e, buy BTC, then transfer that BTC to the recipient.
Ripple isn't making things any easier for such process.
To be more seamless, some entities can even run an currency exchange and take say 5% fee, and provide n-ways conversion service.
There is no need for a "crypto-washed" solution in the equation, especially one that pre-mined 100 billion units of crap and attempt to inflate the value of the craps for their own greed.