And BTW, good luck taking your payment protocol receipt to court when the merchant claims you didn't pay.
It will work for the same jurisdiction, but not cross-jurisdiction. It could even be such that a merchant has to automatically acknowledge if a payment was received via the public blockchain. One example implementation would be forcing the merchant to use a certain address which is attached to the name (the merchant wouldn't be able to generate arbitrary addresses). In effect that's what the DAC ideas are about.
Well, if I didn't know a few people who were told by their lawyers that digital receipts (issued by localbitcoins) would not be accepted as an evidence in court, then I would have also thought like this.
The problem is that our justice system is still in a previous century. They don't know what a digital signature is and they are more likely to accept a piece of paper that came from a printer, rather than a digitally signed file.
I am not saying that no court would ever accept a digitally signed receipt, but I am saying that they are very reluctant to do so.