They reduce drunk driving. One of the things I liked best about living in San Francisco was that I could go to a bar nearly anywhere in the city and get home without having to operate anything more complex than my wallet--either to pay the cabbie or pull out my Fastpass. That has changed considerably here in south Florida. At our last place, there was a reasonable bar within walking distance, but at our current location, the nearest bar requires that you wear leather or Levis as part of the uniform to get in--I'm more of a cargo pants kind of guy.
And I live in a reasonably dense part of Fort Lauderdale, at least when it comes to having a mixture of businesses and housing. We have a grocery store within biking distance (though we don't do that as much as we should), as well as restaurants and other shops, but for most of south Florida, that's just not the case. It's greener to expand public transportation than it is to make more fuel-efficient cars, which is enough of an argument on its own, but if it's safer as well, if we're giving people good options that will keep them off the road when they're planning to be under the influence, we'll have a safer society as well.