I always want people to ask themselves why they are doing NaNo. For some people, it's extremely useful--I think particularly for people who don't have confidence that they can write that much, as you say. For others, not so useful. I won't rule out anything about other writers' process (because writers are fruitbats), but I'd be surprised if it worked long-term for very many people who were writing seriously as a career choice.

One of the hard things for people who don't have the luxury of writing full-time--which is most people--is balancing writing time with household chores, relationship maintenance, etc. And I've seen people for whom NaNo was good and bad that way: they could put off balancing their checkbook while they finished that one novel-length project, but they couldn't put off balancing their checkbook indefinitely. Or their housemates were willing to do extra chores for a month on the understanding that this would not be permanent.