Because your 1 USD on Ripple is NOT USD, it is an IOU (debt) that is under 100% of circumstances worth less than 1 USD.
It is true that counterparty risk reduces the value of the IOU, but the ability to transfer dollars IOUs as easily as one can transfer bitcoins increases the value.
Personally I would be willing to keep all my dollars in the Ripple system because I see the combined effect as net positive.
I would never keep bitcoins in the Ripple system though, because that just adds counterparty risk without adding any compensating benefit.
Unless, of course, certain people are successful at keeping Bitcoin limited to a cripplingly low transaction rate to artificially make blockchain transfers slower and more expensive.
tl;dr:
Ripple makes dollars more useful.
Ripple makes bitcoins less valuable.