The description for the verbose option on the mapReduce command states:

The verbose defaults to true to include the timing information.

When implementing PHPLIB-53, I originally understood that to mean that the verbose option defaults to true; however, that is not the case. Going back to server version 3.0 (in my testing), it appears that timing is omitted from the mapReduce command result unless verbose: true is explicitly provided in the command document.

I also sorted through the history of mr.cpp and found no record of the option ever having a default of true.

SERVER-7649 is the only related ticket I've found on the subject, but examples therein only show an explicit value being used.