OlderBen SaysI keep all of them.
Seriously though I do use a sketchbook a lot. I recently accidentally left it in a friends car and was without it for a couple days and felt completely lost.
Frank SaysSurprisingly, no! Most of my sketching for assignments occurs on loose paper I bind in a 3 ring binder. I don’t find myself drawing aimlessly too much. (Which now that I think about it, seems like a total shame.) I do carry around a small moleskin though, in which I write notes to myself, jot down quick ideas, and every once in a while draw pictures of horses galloping through fields of sunflowers. I also always draw on the tables at restaurants that use paper as table cloths.