I think it's also an easy way to find a ready-made writing community that'll keep you accountable, and for people who find it hard to stay on track, that can be a blessing. I also doubt that it works long-term for many people who write seriously as a career choice. On the other hand, if you're writing seriously as a career choice you probably want something of a writing community the whole year round, and not just in November.

Yes, the just-a-month part definitely can also be a factor. Even if it's not household chores and relationship maintenance--during NaNo, it can be hard to drag yourself out of the novel because it's such a huge thing in your thoughts and your time, and it'd be exhausting to live like that in a novel for more than a month. At least, that's my view of it. (Of course, the living-in-the-novel thing is also partially constructed by NaNo itself.)

But--I agree with you that thinking about why one does NaNo is very likely a Good Thing. I don't think that people who see the website for the first time and go "Oh! I'm going to write a novel!" are necessarily in a place to be thinking about it that way, but it would be interesting to see a post-November discussion that ran along those lines.