New
#10
You've never heard of scratched disks? With a sleeve with one end always open a simple drop on the floor could easily see one little piece of debris get caught in it right up against the side of the disk you don't want anything touching. ut oh! ssccratcchhh... aw crap!
Not everyone has a nice carpeted floor under their station for things like "ESD"! and other factors while running or working on cases. A good impact on the side of a disk extended slightly out of a sleeve can pose a problem if the disk ends up with a crack in the material. Stranger things have happened!
And what happens if you should happen to see something(any liquid actually) spilled on a cardboard which still amounts to paper sleeve? It ends rather soggy fast! At least a plastic casing can be wiped off and even washed clean and dried if that should happen.
("gee? now where did that disk sleeve slip off to I wonder? if I had a slightly larger plastic casing... )