Kelim, Chapter Twenty-Eight, Mishnah Four
1) Covers of scrolls, whether they are decorated or not, are susceptible to uncleanness according to the view of Bet Shammai.
2) Bet Hillel says: those that are decorated are pure, but those that are not decorated are susceptible.
3) Rabban Gamaliel says: both the former and the latter are pure.
Section one: According to Bet Shammai, one might take a scroll covering and convert it for another usage. Therefore, it is susceptible to impurity.
Section two: Bet Hillel says that this is true only of an undecorated scroll cover. It might be converted to other usage. However, a decorated scroll cover is not susceptible because one would take such a beautiful scroll cover and use it for some other need. Note that this accords with the mishnah in 24:14.
Section three: Rabban Gamaliel says that even undecorated scroll covers will not be used for another purpose. Therefore they are pure.