Yeah, I guess if cost is an important factor, trains are not available, and the airports are both sides are on the distant/large/inefficient side, then the bus could marginally make sense. That's not a combination of factors I've encountered in recent memory though (but I don't go to North America much), and it's so marginal that I think every time I'd go for the increased comfort of not being on a bus (no matter how "luxurious") for 12+ hours.