First: bitcoin will never be used for point-of-sale transactions. It requires a minimum of 30 minutes - 1 hour (depending on the vendor's level of paranoia) to verify transactions and such a payment delay will never work in a point-of-sale setting. Lots of people propose ideas to try to get around this but they all require vendors to accept unvalidated transactions, which is equivalent to taking someone's word that they will pay
What do you think credit cards are? Unconfirmed Bitcoin transactions are still several times less likely to fail after the customer has left than that. If you are trusting enough to take credit cards, you might as well take unconfirmed
valid Bitcoin transactions as well.