The burden of proof is on the person attempting to the contract, and "the ripple ledger says so" is not going to overcome an alleged debtor's claim they've been hacked and never entered into any contract in the first place.
And
exactly the same thing is true with Bitcoin payments, so it has
nothing to do with Ripple. If you're dealing with large amounts of money, write things down. Use purchase orders and invoices, perhaps in electronic form, perhaps written down on paper, perhaps both. If all you want to use Ripple for is as a distributed Bitcoin exchange, then enter into a written contract with a reputable Ripple gateway. Bitstamp is one of the more popular Bitcoin exchanges, and it's also a Ripple gateway. That's just one written contract with one registered business.