I believe that NaNoWriMo is good for a challenge. Just to be able to say that you popped out a 50,000 word novel in a month is quite impressive. You feel accomplished after you've done it. But aside from that, I believe a novel takes time to write. Time, proper planning, proper revision, all that really can't be properly done in a month. I like to submit my best, and although perfection isn't a requirement for the contest, I'd rather be able to submit something as close as I can to that.