I agree about the interface. I was mostly referring to confirmation time. When you say in practice Bitcoin confirmation time isn't a problem are you speaking for yourself or everyone?
People are conducting transactions using Bitcoin, and complaints about transaction times only arise in a tiny minority of instances, therefore transaction time is not a problem.
In addition to this, Bitcoin confirmation is faster than any other means of online transaction,
when you compare like to like. An "instant" PayPal transfer is reversible for at least several months, and probably indefinitely. Bitcoin transaction also show up within a matter of seconds and are typically irreversible within an hour.
Assuming equivalent UIs, what theoretical advantages to your third party off-chain service provide have that zero confirmation Bitcoin transactions don't already provide?