Atlanta resident here. MARTA is a joke, yes. It doesn't go to too many places, and it has a racist nickname locally because its most often used by minorities(Moving Africans Rapidly Through Atlanta), but MARTA suffers mainly because the Atlanta metro area that you showed is actually an amalgamation of various towns and cities. Of the Perimeter(that big circle), Atlanta takes up only a third or so. There's other cities in there too, like Decatur, Panthersville, Chamblee, Buckhead, Vinings, East Point, Cumberland, and so on, and many of these either don't want mass transit or can't afford it. Hell, take a quick look at this map:
That is just the stuff in part of Dekalb County, roughly a fourth of the land in the Perimeter. Hell, there's even a ghost town in the Perimeter: Constitution, GA.
Also, that map is only for the MARTA train system(though it pretty much shows where I live. I'm down next to King Memorial station). The bus system is actually quite impressive and extends well beyond the Perimeter.
As for mass transit versus highways...Atlanta needs both. Mass transit for those of us living inside the Perimeter, but Atlanta is also a major transportation hub and has a lot of materials being moved up from the coast and distributed out to the rest of the South or the country as necessary. So neither will necessarily work by themselves. We need both an improved highway system for the tourists and shipping industries(especially with Savannah's harbor being dredged and expanded), and an improved mass transit system that doesn't have racially or economically-biased stigmas.