I think the point is that Paxum and other banks won't deal with centralized exchanges, but their business is to move money between individuals, and if you use this system, they won't know if you're trading Bitcoin or just sending someone money. There won't be a single point of failure, that being the exchange's bank account, since many accounts will be involved individually.
Chargebacks will still be a problem with some systems, and I'm not sure how to get around that (force PayPal to send as gift? Use services that don't do chargebacks?)

As for using something like ripple, how do you get your money into ripple? And if it's a hassle, why bother instead of getting it into Bitcoin more directly?