Bitcoin is a nightmare for both buyer and seller due its volatility, not to mention the difference between exchange rates. You may buy your bitcoin with a specific price but the merchant/payment processor values it differently(lower). Just check bitpay to see for yourself
Cash payment is an obvious counter-example. Yes, the seller needs to keep records for tax purposes. But the buyer does not.
For an example of a crypto payment system (though not crytpocurrency) that attempts to provide anonymity in a way that doesn't make governments unhappy take a look at GNU Taler.
