Littermates vent their energy on one another, taking some of the pressure off you. Double vet bills. They are NOT identical twins. A 6 year-old dog produced from a mating several litters ago is just as genetically similar to a pup than any of its current littermates. Linebreeding is different than inbreeding.
Line breeding is where the sire and dam have several of the same ancestors as each other.
Inbreeding should never be attempted by anyone other than the most expert of genetists and breeders. Linebreeding is not always bad. Double recessive traits are expressed far more often in closely related matchings than in outcrosses or in unrelated crosses. Personally, I think linebreeding is altogether unnecessary at this stage in the game. The traits of the Border Collie can be maintained and improved through directional selection just as well.