[absconditus]

This is exactly why I do not believe that what we do is engineering. Engineers do not refuse to use proven techniques and technologies because they are "too hard" and instead use easier methods that have serious shortcomings.

[nakkiel]

How about the freedom/need to experiment?

If MongoDB is popular, it's because it has some compelling features/arguments (I'm not sure of which ones exactly). But classical RDBMS seem to be trying to close the gap.

[Detrus]

If you're taking your time to design some important large scale system, using proven technologies is easier overall with higher up front costs.

Prototypes are another story. You'd optimize against mental overhead.