My general metrix for "what should we make tax paid (free)" is anything that will benefit all of society more than it costs us.
The benefit of people moving to public transit alone, and the benefits to our health, I think that alone makes the argument well enough. Add to that the reduction in drunk drivers, the fact that homeless and poor people will have transport accessiblity for interviews and anything else they need... it just makes sense as a ROI on every level. Even the people who wont directly use it benefit by having a cleaner environment and fewer poor people or people on welfare.