Why would you get a cable with BNC connectors and then add RCA connectors?
I have been building cables since 1996. What I discovered is that it is more important to properly terminate the coax to the connector than what type of connector it is, RCA or BNC. RCA connectors are not designed to be terminated to every type of coax with minimal impedance discontinuity. BNCs are.
If the optimum BNC for the particular cable is selected, the cable can be terminated with minimal impedance discontinuity. There is not just one type of 75 ohm BNC connector. There are hundreds of them, depending on the cable.
The BNC-RCA adapter creates a minimal impedance discontinuity compared to terminating the coax directly to a RCA connector. Most customers end-up using an RCA adapter on at least one end.