I generally book tickets months in advance. Quite often, due to a change in circumstances, I realise that I am not going to be on the booked flight.
At the moment there is no incentive to tell the airline (low cost) as there is no refund.
If the airline were to make a gesture, such as a small credit against a future flight, I would let them know I was not going to be on that flight and let them rebook it.