Thank you for taking the time to respond in kind; I do not disagree with what you have stated. I disagree with choosing this by default; it violates the principle of designing tools and APIs for use by the general programming public in such a way that they fall into the "pit of success". http://blogs.msdn.com/brada/archive/2003/10/02/50420.aspx
To me, the choice of performance over reliability is the hallmark of mongodb, for better or worse.
I agree with you, incidentally. I think it might be a better design decision to be slower (but more reliable) and to fail loudly by default (10gen has started down this path a few versions ago, by turning journaling on by default). It's messy, but messes get peoples' attention, at least.
That said, I think that people really do overblow the issue and make mountains out of that particular molehill, because all the tools are there to make it do what you want. Many times, it comes down to people expecting that MongoDB will magically conform to their assumptions at the expense of conforming to others' assumptions. Having explicit knowledge of the ground rules for any piece of technology in your stack should be the rule rather than the exception.