I've never liked sketchbooks. As an artist, you get a sketchbook for every art class. It's an obligatory teaching gesture: "You might use this at some point, and it really makes art class seem like it has non-art-class homework." You're told to be creative. To be fluid. To not think of it as a sketchbook. More like a diary, a way to get thoughts out. Use and abuse it, because it will be a jumping off point for your "real" art. But the reason I have six sketchbooks - some with only a few used pages - is not because I develop my ideas or experiment with ideas. I simply feel obliged to use them.
When a person uses a sketchbook, an inevitable narrative develops. Because the sketches are bound together, they affect each other. When I use a sketchbook, the narrative is flustered, missing several chapters, and written with a bum hand. Seeing a flurry of empty pages flusters me. I start something, only to wish it were on better paper. I'm cramped by the fixed nature of the book. Trying to accomplish anything in those pages dooms the drawing to necessary incompleteness.
Many artists I know use their sketchbooks to jot down ideas and compositions. I find this to be a laborious extra step. I take an idea straight to the full size paper and free it from a cheap binding. At the very least, I use newsprint to work freely and aggressively. The artists who consistently fill moleskines with doodles and studies, I feel, are precisely the ones who need a sketchbook the least: they already have the ideas and energy to be working full size.
Of course, sketchbooks aren't like this for everyone. I believe a lot can be learned from artists (I'm thinking Da Vinci and his folios) who turn the sketchbook into an art form. But if you ask to see my sketchbooks, I will either politely decline or carefully select a few worthy sketches while making excuses for the rest.