Kansas City buses are like that. Outside parts of a handful of routes they're only tempting if you have way more time than money. Making them free does help the people already using them but I can't imagine it'll do much to increase use of it for actually getting from place to place (increasing its use as a place for the homeless to get out of the heat/cold, as mentioned elsewhere in the thread, might happen, and isn't exactly a bad thing, though I agree that if it becomes common it might actually reduce total ridership)