We looked at putting all four kids on hers but I needed one kid to be able to max out my HSA. The cost of adding my son to the plan was only a couple hundred per year but allowed me to save like $3400 more over the year. I remember about 23 years ago we were double covered and just about everything was paid for (expensive fertility treatments, etc) due to the secondary picking up the remaining costs. It sucks that it doesn't work like that anymore.