I'll be honest and say I didn't read through the whole topic, so I may be just regurgitating something someone else has already said.
I think the key issue here is not that so-called "poor" media is so popular. After all, what makes it poor? The shallowness, the easy reading, the lack of serious, deeper thought-invoking narrative? The presence of cliche story elements? It's safe to assume that this is really what we're discussing here. Does that make it "poor", no, I wouldn't say so. I'm sure we all enjoy from time to time the hollow husk of easy to consume media. That begs the question, why is shallow, easy to consume media that lacks serious depth so popular amongst the majority of the population and often leads to the exclusion of their counterpart? Unfortunately the answer is clear. That's not what people want. It's not what the average person seeks out or enjoys most. People can argue about what makes a book good or bad but it doesn't change the reality of it. I think we can all agree that the average consumer isn't looking for depth and carefully crafted and meticulously edited media for their consumption, they want to be spoon-fed a roller coaster ride of emotion, even it's cliche and narrow. Think about how popular mindless action movies are, and how everyone in this topic has probably enjoyed one at some point.
It's not the idea that people enjoy poor quality media that I find repulsive, it's the fact many people exclude the more refined books or movies that require some effort and thought from their lives completely. Hell, it even applies to videogames. I can only dream of a world where people prefer turn-based strategy to killing zombies. I tried to get a friend into one and after about five minutes he said "this is boring" and never tried it again. Likewise, I doubt a Twilight reader would sit through Romance of the Three Kingdoms or Atlas Shrugged without yawning and wishing it was over.
Then you have the types that are repulsed by this cliche, shallow experience. Is a book bad if it accomplishes what it set out to do? I wouldn't say so. But let's not obfuscate the point or get all high-brow about media like Twilight. It is what it is. Don't try to put it on an equal artistic or literary level as actual great works. And it's unfortunate this type of book is the only thing many people will read or care to read on their own time. We don't need to take some relativistic view and pretend all works have equal merit, or that my scribble on a piece of paper is on par with Van Gogh, because that's really where that line of thinking leads. I enjoy a nice cliche fantasy or sci-fi novel now and then, but I also agree something with more meat and substance to it is preferable.