"you can't tell who paid for what" Actually you can. Vendors who do this require payers to use a built-in functionality in most Bitcoin wallet apps to cryptogr

"you can't tell who paid for what" Actually you can. Vendors who do this require payers to use a built-in functionality in most Bitcoin wallet apps to cryptographically sign a message identifying them. However this is not very user-friendly, does not work when sending from a hosted wallet (no access to the private keys), and reduces anonymity for the vendor since everybody knows his main payment address.