Joining a loyalty program in this industry usually comes with low to negligible costs, so joining does usually make sense. Whether or not it makes sense to change flight purchase habits because of a program is question that should be asked, but a lot of people -- including "kettles" -- make decisions based on "loyalty" even if it may be to their immediate material detriment to some degree.

For example, there are lots of passengers on flights from the US to China and India who pay a sort of premium and go with their US carrier of choice even if it has a marginally higher fare because they want to collect miles in their program of choice. Some of these decisions based on the loyalty program may be irrational, yet they are often made to the benefit of the airline program operator despite the passenger's higher cost for "loyalty".

Even for domestic-only "kettles", it is quite routine for those who book them to be told they have a preferred carrier whose program they want to use for their paid flights -- it results in a benefit to the airline program operator as well since these customers often end up on their choice program's airline operator rather than on the competition.