If it remains unconfirmed for an extended period of time, you won't be able to spend it and neither will the customer be able to spend their change.
If the merchant or the customer wants to spend their output, they can broadcast a child-pays-for-parent transaction and incentivize the miners to include both.
The volatility of the fees in recent times hasn't been very kind and there is still a good enough possibility for the customer to be able to push it directly to a mining pool, RBF or not.
It really depends on the amount transacted. I, as a merchant again, wouldn't be bothered to accept such unconfirmed transaction for an amount less than $300. I don't believe that a customer would choose defraud me that way, I find it difficult thing to happen, there's still a decent percentage of uncertainty and I'd, either way, also accept credit card payments which are far easier to reverse and whose finality takes about 6 months more than a bitcoin transaction does.
Furthermore, "the customer is always right". If he asks for unconfirmed transactions, I might dissatisfy him by disagreeing, which is definitely not a smart move.
For thousands of dollars worth of bitcoin, you should absolutely require at least 1 confirmation.