There are a few times that I will take public transportation.
Living near the northwest limit of the Chicago area transit system, it is convenient if I'm headed to a convention at McCormick Place. But, I haven't gone there in over a decade... The conventions I go to moved out to the suburbs, where use of public transportation is comical at best.
Going to one this past May, I checked the PT route vs. car. Car took 1.5 hours. PT route involved 10 minutes to train station. 1.3 hours on train. 20 minutes on another train. 10 minutes on bus. 19 minutes walk from nearest bus station to convention site. Cost for round trip? More than the cost to drive, including the $13 parking fee at the convention center.
"Free", as pointed out by others, is only to the rider... Someone has to pay for it. In Milwaukee, they're going to build a trolly system in the downtown that will be less cost-effective than simply renting a bunch of limos and drivers to do the work. And that's if the ridership reaches their claims... but the buses currently plying the route don't meet those numbers!
Does PT go where I shop? No. Does PT go where I work? No. Would I use it if it was free? Why?