The bottom line is that MongoDB and MySQL are two different persistent data structures. MySQL is a more powerful data structure that can do more things. MongoDB is less powerful, but is more efficient at certain things. Due to pre-mature optimization or shortsightedness, some folks are romanticized with the efficiency of a less powerful data structure (MongoDB) and fail to realize that their application really need the more powerful relational data structure.

These things should be good learning examples for all.