I used to check my small overnight travel bag (8kg) in with my suitcase (15kg) and carry onboard just reading glasses, book and a bottle of water. Qantas recently stopped that by working to rule and charging per piece of checked baggage. Now they want to charge me $ 90 to put my overnight bag in the hold with my suitcase, despite the total weight being within the luggage weight limit. Needless to say, the bag now travels in the cabin.
Charging checked luggage by piece encourages passengers to put baggage in the overhaed lockers - where it doesn't belong.