Issue of preference.
I believe (but haven't checked) that they are required equipment for most sanctioned offshore racing events. Like many bureaucratic requirements, this is probably included more to protect the sponsoring organization rather than because it is based upon an objective study of the efficacy of the lines as a safety device.That said, I have, and will continue to have, double lines aboard. In over 35 years of sailing they have kept my bum on-board countless times. Not because they have caught me during some wild tumble. Simply because, oftentimes, it only takes a light touch to correct an incipient loss of balance and keep it from becoming a tumble or the preventing the beginnings of a slip from becoming an inundating slide.On my last boat, a Cal-27 T/2, I replaced the white vinyl coated lines with clear coated lines. I really liked having the ability to visually check the condition of the lines.The obvious rusting at the ends has convinced me to go with uncovered lines when I replace them on our current boat. Stainless steel just doesn't tolerate a wet, anaerobic environment.In support of the comments that suggest ditching the lines altogether. They really are unlikely to stop a falling body from going overboard. I try to never rely on them for this purpose, and always try to keep 'one hand for me and one for the boat'. When single-handing or in rough weather, I use and require harness, tether and jacklines. There is no substitute for common sense. It doesn't take a genius to look at those spindly wires and stanchions and realize that they are, for the most part, merely a security blanket. Security blankets are helpful but not to be relied on.