Casual fans are one problem, hardcore fans are a problem too - they can alienate the casual fans by not having enough episodes to keep them interested until 2010, and at the same time alienate some of the more hardcore fans who would otherwise have stuck with it and waited for S4, because quite a respectable fraction of the fandom seems to be of the opinion that the show is going to fuck up dealing with the lack of Tosh and Owen, and aren't expecting S3 to be great. And a five-episode mini-series may not be enough time for the producers to pull it out of the bag and convince people that hey, this is actually going to work out okay.