You my friend are wrong. Everyone is completely capable of getting their nutritional needs through the right diet. Now does anyone get them? Mostly not because people are lazy, but seeing as we have mouths to speak and are smart enough to determine to what we need to be 100% healthy we can perfect our diet. And there are people out there, such as Ethiopian kids who live a full life (not a healthy one) but live a full life by just meeting the bare minimum. So yes for as much as we know, snakes do not need supplementation and can live a full life just on feeders, but who knows if by giving vitamins will they live longer? brighten up in color somewhat? produce more eggs?