The question is not whether you can write 50,000 words per month, it's whether or not you should.
The value of NaNoWriMo is twofold. First it brings writers together in a common project. The value of this should not be underestimated. Second, it gets writers writing. No small feat that.
It's easy to find supporters of NaNoWriMo all over the web. It's a developed and exciting community, and sometimes when you question a community movement you get hammered by its supporters, but I'll risk it.
Considering the speed of publishing today, we all feel the pressure to write faster so we don't get left behind. The market changes rapidly, especially with the advent of eBooks. The more we write, the reasoning seems to go, the more money we can make.
But in the race to be rapid, I wonder if we're sacrificing the power of our writing. The ability to create compelling, emotionally moving writing takes time and reflection to develop. It requires that we look deeply into ourselves and the people around us before we start typing.
This ability takes time to take root within us. It begins small, like a seed, and cannot be forced to grow beyond its time or it will wither and die.
I believe most writers have the ability to be great writers, but they lose their chance by writing before they think.
One powerful novel is worth ten mediocre efforts.
If you want to read an examples of a writer with mature, developed power read something by James Lee Burke. I guarantee you'll be impressed and put the book down thinking maybe it is worth it to take the time to think, really think about what you're writing before you put it to paper.