If a rabbit is registered, it shows that it has been examined by a Registrar and conforms to the standard for the breed, with no DQs. If all of the rabbits on the pedigree are registered, it gives buyers confidence that they can expect the same quality out of a rabbit purchased from you.
I was talking to a judge about this, and he says that in certain breeds that are prone to a sweeping problem throughout- such as not making Senior weight- that it is a good idea, while in others, the breeders/exhibitors don't care.
The same goes for Grand Championships. For instance, in Satins (here on the west coast, anyway), you almost never see a Senior rabbit on the table, and thus there are very few that earn a GrandChampionship. They show them through Intermediate and then feel they should be home being bred. Sweepstakes points are a big part of the "reputation" of a Satin breeder on the national level, whereas on the local level they judge breeders simply by the rabbits themselves.