What happens if I "receive" a Casascius coin in RL and I want to exchange it for the real asset but the person who made it already took the money from it - or worse: runs a hidden node that will double-spend with a high fee any coin that I try tro move off a Casascious coin so I have no way other than trust to spend the BTC?
The problem you describe happened already dozens of times in Bitcoinland and it always ended in tears and "hacks": mybitcoin webwallets, pirateat40's BTCS&T, GLBSE, Bitcoin-24, Tradehill, inputs.io...
Ripple is NOT designed to save you from your own stupidity (of trusting someone with your funds who doesn't deserve it), just like with BTC you can build insured systems on top of it, but the basis is still that you need a trusted counterparty to keep your assets, just like any centralized exchange that you use (or physical coin manufacturer...).
The problem Ripple solves is that to actually use Bitcoin in most cases, you need to exchange BTC for something else. Ripple has a use case far closer to BitPay than Bitcoin, on Bitcoin you can ONLY receive BTC while on Ripple you can receive both XRP and anything you trust to be redeemable. Yes, the latter can be exploited (just as it has been in bitcoinland for years) however it can also be put to good use, as you see from the amount of great enthusiasm people have towards Coinbase and BitPay, who are in the end centralized closed source services that need to be trusted by both merchants and users to keep their payment data safe (BitPay would be THE ideal honeypot for CIA/NSA/GCHQ!) and to actually process transactions and pay out as advertised.
Ripple takes this concept one step further by decoupling merchants, asset holders (=gateways) and payers by introducing open markets for different assets and introducing market makers (the "traders" on Bitcoin exchanges BitPay sells their coins to).
I don't get how people don't understand this after 1-2 hours of reading and still claim that Ripple is so complex, impossible to understand or anything like that, on the other hand it might be a large intellectual feat of realizing that any service that you use in bitcoinland that does NOT allow you to privately sign transactions with your own local key offline that you choose yourself is something that is a trusted service and could also work as Ripple gateway with the added benefit that balances would not be locked in on that platform.