As late as I am...
Microsoft aren`t stupid. Most people have no reason to actively turn the sharing on. And so - almost no-one will. A feature they spent money and time making will go to the trash.
Some others (the less... adventurous) don`t even go to windows settings and need such options to be turned on by default *cough* grandma *cough*. A wizard at first startup might be an alternate solution but that would likely just confuse those people.

Also, watch this (I originally found this video here on DC a while back. Courtesy of Mouser, I believe): http://www.ted.com/talks/dan_ariely_asks_are_we_in_control_of_our_own_decisions?language=en

The moment Microsoft set this option as the default, they assured it will be 90%~ active. People are like that.