True, but it is possible to still accumulate mileage over the years to gain an award ticket, by carefully choosing airline programs to maximize coverage of airlines through alliance or mileage partnerships, even if you choose airlines on every flight without regard to mileage accumulation.

Also, even if you expect the mileage to be completely useless (the airline is an isolate as far as mileage partnerships go, and you do not fly it frequently enough to ever collect an award due to mileage expiration), being a member of the airline program may confer a slight status elevation over a not being a member. Once, as a General Member (i.e. "no" status) with United, I got a free Economy Plus upgrade at check-in. Apparently, the flight had so few United Mileage Plan members of any status on that flight that they gave away Economy Plus seats down to General Members in order to open Economy (not Plus) seats for other passengers who did not yet have seat assignments or something like that.