Sketchbooks are technically meant for things like rough sketches. One good reason not to do elaborate, final drawings in your sketchbook is that, if you're really happy with them, you'd be tempted to tear them out and frame them or something like that. That's especially seems like a dilemma if you're using a hard bound sketchbook. There might not be anything wrong with tearing pages out of your sketchbook, depending on how you look at it, but some people may want to keep the pages in the sketchbook.