I keep them because I like them. I am not knowledgeable on paper collecting so for me it is the cool factor - I just think they are a neat thing to find.
They are modern today, but wont be down the road at a point. Will they appreciate much in value - well that has a lot of variables so who knows. However, there was a time national notes from 1929 were "modern" as well. So for a 1-2-5-10-20 star I will keep them and pass them on down that road in the thought that one day they will prove valuable in inspiring a descendant to take up the hobby. More than that I am not overly concerned about.