Recently, I was drawn into a discussion about bad books and the policy that some people have of returning them. Personally, I've bought thousands of books over the years and never returned a single one. Moreover, even when a book is absolutely horrid, I generally make it a point to slog my way through to the end once I've started. (There are maybe three books that were so bad that I couldn't finish them, but in each of those instances at least part of the reason for my failure to finish was that they were actually library books that were [thankfully] due back.) From my perspective, the bad ones that I own add to the diversity of my collection. However, the more I think about it, the more it occurs to me that reading bad books can actually be good for you from the standpoint of being a writer.
First of all, books can be bad in a lot of ways. From cover art to editing to plot development, there are a million ways for the written word to leave a terrible taste in your mouth. That said, I think that reading awful books can be a learning experience, because there's a lot they can teach you. For instance, a book with a bunch of typos will probably cause you to associate the author's name with crap. Assuming you don't want the same type of reputation with respect to your own work, reading such a book will teach you to never settle in terms of your own writing; you'll realize the value - and necessity - of proper editing. It's sort of a bookish version of Scared Straight:
In short, there are some places that you just don't want to go with respect to being an author, and reading bad books will firm up for you many of the things you definitely do not want when it comes to your own writing, whether it be bad storylines, underdeveloped characters, what have you.
In addition, reading terrible books can be help you by being an inspiration, after a fashion. Have you ever been to a movie and left the theater thinking, "Egads, what a crappy film! I could have made a better movie than that - and for less money!"
Bad books can provide that same emotional spark. If some hack can get a traditional publishing deal and sell a bunch of books that are basically crap, surely you can eek out an existence as a writer of quality material, right? (It may not necessarily work out that way, but we're talking inspiration here - something to keep you motivated and enthused while writing - not the end result.) Thus, every time you start to doubt, you can look at that awful book and - knowing that you can do better - feel encouraged.
In essence, I see reading bad books as a character-building exercise, the literary equivalent of having to eat your veggies. I'm not necessarily saying that you have to go out there and actively seek out bad books - one or two (quite likely more, if I'm being honest) will eventually cross your path in the grand scheme of things - but don't just toss them aside as if they have absolutely nothing to offer.
They say that experience is the best teacher, that you learn best from your own mistakes. However, I say that the best student is the one who can learn from the mistakes of others. Learn from the bad books around you; they can help make you a better writer.