The problem is that you need an account to hold BTC in the Ripple network. There is no known practical way to allow Bitcoins to transact directly on another network without a central authority to hold the keys that release the Bitcoins on the Bitcoin blockchain.
Hopefully, this should only be an issue for a short period of time. Once billions of XRP have been given away, there shouldn't be any significant difficulty in getting enough XRP to open an account.