Probably the most foolish two words a ten year old boy can mutter in the presence of his mom on a summer afternoon... At our house, if you didn't have your wits about you and decreed such a statement out loud, it guaranteed time in the yard pulling weeds. Artists should never be bored. We should always be working on something -- be it a commission, heading outdoors to paint, cleaning the studio, promoting our work or filling up pages in the 'ol sketchbook. Of course when at all possible draw from life, but if a live model is unavailable, doodle off the top of your head or from photos in the magazines lying about the house.
Like painting, it's about the mileage -- whether it's a successful painting or not, you learn and grow from each experience. With sketchbooks, you're honing your drawing skills, recording ideas & reference and providing yourself a journal of progress all at the same time. And it's a personal thing -- doodling in your sketchbook can be a relaxing past time because there are generally no rules, no deadlines and no one looking over your shoulder (unless you're doing it in public).
A couple of famous painters both told me, "Work on 'starts' and don't worry about the finishes...they'll come on their own". That's what drawing in sketchbooks is like -- a lot of starts that ultimately contribute to my progress as a painter. So if you ever feel bored, draw -- it's the foundation of everything you create...and it definitely beats pulling weeds.