Yes, publishers often select and underwrite titles for a variety of reasons that have little to do with quality. They buy up the rights to titles (that are usually ghostwritten by professional writers) from celebrities and jump into hot markets with "copycat" titles, such as the endless stream of vampire-related books that followed the success of the "Twilight" series. However, they also impose basic quality standards on their writers, and they (usually) have standards about what they will and will not publish. To a knowledgeable consumer, seeing the Random House or Farrar, Straus and Giroux name on the spine (to take two examples) says that they're likely to get a well-written, well-edited book that's not going to be a waste of their time or money.
In the new era of self-published books and eBooks, most of the functions of publishers can either be farmed out or are irrelevant. Editing, copyediting and cover/book design can be contracted out or done by an experienced writer. eBook conversion can also be done by the writer or by a contractor. Printing can be done by any of a variety of companies. Distribution can be done by the author for eBooks; distributing print titles is more difficult, but can still be done through companies such as Ingram's Lightning Source, which deals with most of the world's major booksellers.
However, the one thing that neither a self-publisher nor companies that assist self-publishers does is curation. The writer of a book is the last person who can make an objective judgement about its quality--for better or worse, most authors are either far too hard on themselves or are deeply emotionally invested in their work. Self-publishing services companies are concerned with generating as much revenue as possible from self-publishers. That means not turning away any manuscript, no matter how poorly written, so long as the author can pay for their services. About the only thing that will keep a title out of Amazon's and Barnes & Noble's self-published eBook collections is if it's proved to be largely or wholly plagiarized, and even that doesn't happen very often.
Amazon and Barnes & Noble (and other booksellers) claim that their customer reviews provide a curation service for customers, but the reviews can be gamed:
- Authors can encourage their friends and acquaintances to post positive reviews, or they can pay people to do so.
- Consumers sometimes give extremely low ratings to books because they believe that they're priced too high (often, the consumers giving the ratings have neither purchased nor read the books).
There are also book curation websites, but none of them are widely popular, and they can be gamed the same way as the eBook retailers' sites. That leaves the tasks of curation and quality control to the publishers. To the extent that publishers abandon those roles, or de-emphasize them in favor of chasing celebrity and copycat titles, they'll give away their biggest advantage over self-publishers.