Thanks for all the interesting replies.
I hear what people are saying. I tend to agree, it is weird when other people take over for a writer. Yet, at the same time, a book is a book no matter who writes it. The novelization for "Star Wars," as an example, was actually not written by George Lucas, even though he was credited as the author...nonetheless, that is a great novelization. It helps that it turns out that Alan Dean Foster wrote it, a man who is an accomplished scribe.
I'm going to perhaps almost assume, even though just about every source -- save for a few blogs which seem to report this as fact -- seems to say otherwise, that a lot of R.L. Stine's stuff was ghostwritten. It just doesn't seem possible that someone could have produced all those books during the timeframe they were released. At the very least he must have had an assistant do the polish drafts. I will point out that there was an author who indicated that he wrote, I believe, a Fear Street book, and there was a brief series where I believe co-authors were credited (I just looked up the author who mentioned his ghostwriting gig, it's Tom Perrotta who wrote "Election"; you can look up the source, it's an Entertainment Weekly item written by Missy Schwartz, dated October 12, 2007). I should mention that R.L. Stine does say he has produced every word. And I'm sure a lot of his earlier works were in fact done by him alone, including many of the Goosebumps titles (by the way, I tried to write a Goosebumps-type book once, but found the one-sentence paragraph structure very daunting).
I'm sort of in love with the concept of starting a series and then having others take over. I would love to be able to simply sketch out an idea on paper and then have it packaged by someone else, sit back and collect the royalties. Someone like Patterson must have a field day; I'm sure he still writes some of his own books, but the co-authored ones must really pad his income, it's like splitting yourself into additional productive pieces. And I'm sure it's not unlike Hollywood TV shows, where someone creates a show and then eventually leaves the job: someone else takes over as a showrunner, with a group of writers, while the creator still derives plenty of residuals. Like Carrie's Younger Brother said, it's the development of a brand, a funding of a start-up to some degree.
Patterson says he writes 80-page outlines to the books the co-writers write (if I recall correctly). I'm not sure I believe that, or if they are 80 pages, they must be short pages (I would hate to be the hired writer stuck with an 80-page treatment). If I were Patterson, I would just write a three-page outline, hand it off, let the co-writer run free, and go get some pizza and enjoy the rest of the day.