First of all, the market is already flooded with crappy books. NaNoWriMo (a challenge that only a small percentage of people who start finish) isn't going to make a huge difference. But, most importantly, who is to say what's crappy? The world's flooded with what I think is crappy furniture, for instance, but someone else might love it. Now, I would never encourage anyone to throw an unedited first draft into someone else's hands unless that person is editing it, but if you truly work on your piece, it will be judged fairly.
Second of all, three of my four writing buddies are formally trained in writing. And, the one that isn't has a real natural talent. We're all aware of our shortcomings and intend to work through them before our books ever see the light of day (if they do). The people I'm working with know that they will not have a finished piece on November 30, and people who I've talked to who have finished have been working on their Nano novels for over a year now and haven't sent it to a literary agent. Chances are, their pieces are far from the draft they wrote during the November they participated in Nano.
Finally, if I do decide to send this book to literary agents in the distant future, I'm not forcing them to read it. In fact, I'd probably have my mind blown if they did considering they don't read most. So, don't read my book. Don't publish it. I write because I love to write and feel that I have a story to tell. I don't feel that my creative process hurts anyone.
Yes, NaNoWriMo's tag line is "30 days of literary abandon" and anyone who knows me at all knows that literary abandon sickens me. Poor grammar and a lack of respect for the art of writing are my major pet peeves. BUT, putting perfection aside for 18 days so far has resulted in almost 100 pages of work for me. Some of which will be deleted but some that I'll develop and make better. Then, I'll make it even better and maybe (just maybe) send it to an agent. If this helps me be a more creative person and do what I love, why knock it? Amateur writer or experienced writer, having a goal to work toward (no matter how crazy) makes a difference. I haven't done my usual page deleting since October 31 and for me, that's progress.