It's something the Album and Folder manufacturer idiots did and for some reason, everyone falls in line with them. As for the
Lincoln Cent Albums. Imagine if those people started to add slots for all the odd ball cones. Not just the 55 DD but imagine slots for 74 Aluminum one. A slot for the 59
Wheat cent or the many 43 Copper ones. Then too get a book on Lincoln Cents and you would see there are odd ball types for almost every year. If those manufacturers have it there way we would all end up with Albums 5 feet wide.
NO, a set is complete when you want it to be.