The advantages and disadvantages of Pay with Miles has been hashed out here. However, if you are thinking of getting rid of your existing Delta Amex card because of the annual fee, and losing Pay with Miles is a consideration - you can keep the Pay with Miles option by switching to the free Delta Skymiles Blue Card.

Confirmation that Pay with Miles is a benefit of the Blue card: https://www.delta.com/us/en/skymiles...pay-with-miles