And that's the point of sketch books, they're for everything that occurs to you visually, conceptually, shopping list-ually. Drawings range from the hideously crappy to the barely legible, but it's a vital part of the process. Sketch books that are designed to be looked at by others aren't really doing their job properly, like beautiful lingerie that doesn't support your boobs and gives you a wedgie every time you sit down. You take my point.
I love coming up with ideas, shots, images for a film and having a sketchbook to hand means that I can jot everything down as it enters my feverish brain. Of course the embarrassment factor of drawing in public can be a tricky hurdle to get over, but I find that headphones give me a bit of protection.
Some people's sketch books really do look genuinely fabulous, but they are genetic anomolies, like Elle McPherson (who apparently always matches her bra and knickers).
In the light of this I really shouldn't put my sketch book images up here, but I'd like to point out that they are heavily edited for taste.