I signed up for Delta's frequent flyer program because i frequently took cross country trips and decided I needed to take full advantage of the fact that only one airline provided a non-stop route from my location to this destination. Being apart of this program led me to choose international flights with this carrier over others because I would earn miles.
Once another airline with cheaper fares took on this route I completely abandoned optimizing for miles and simply started optimizing for cost like many travelers do.
Miles take too long to earn for travelers who fly less than 5 times a year, especially if these flights are domestic. If I were guaranteed to earn at least enough points for pay for a round trip around 500-750 miles from my nearest airport in exchange for flying with one airline consistently I would definitely consider it.