I still have my first sketchbook ever, my freshman year sketchbook, and the doodle sketchbook after that. I'm keeping them, because my car has a big trunk. I've got 2 more random ones and I just bought 2 on sale for school.
It's mostly for space consideration. Sketch books take up a lot of room, and I fill them up fairly quickly (since I doodle a lot when I'm doing other things, like watching TV or reading). I just don't have the space to keep 'em all, sadly! So I keep the ones that I really like in a small portfolio case. I give a lot of sketches away, especially to the little kids at the school where I work. They love stuff like that! Only the really sketchy, abstract stuff gets thrown away.