In the increasingly split world of air travel, where only elite fliers have access to better (even just aisleorwindow) seats, shorter lines, and free baggage allowances, it is more important than ever to accrue these miles.
Priority boarding is also a big draw, because Southwest's chaotic no-assigned-seats system can be a turnoff for travellers unwilling to push to the front of the line or throw a few elbows for a window(or aisle) seat.