I don't know how it could be unethical if the airlines are suggesting it.

For example, I wanted to book award seats to Hawaii. I managed to get one for my date, but I could only get the second one outbound. Nothing was available inbound.

Since I had a ticketing deadline that could no longer be extended, the airline suggested I book the return on any date that it was available and then change it if/when the date I needed became available. There would not be any cost to me if the itinerary would be exactly the same. At worse, I would have to cancel everything, pay the redeposit fee and buy a ticket.

Essentially, I was holding a ticket that I had no intention of using at the suggestion of the airline. In the end, my waitlist came through and I changed the ticket.