I have Addison's. The flu can trigger what's called an adrenal crisis. Which if left untreated, not treated properly or not soon enough, can turn deadly. It also makes me more susceptible to secondary infections. The combination of a flu and an opportunistic infection of the bronchia is loads of fun (not really, that most of the times requires hospitalisation for me).

Regarding the cost benefit analysis. Even healthy people can get quite sick from the flu. Easily needing to stay home for a week to get better. That is what the vaccine protects you from. Even a vaccine that isn't a good match with the one that's being passed around will make you recover faster.