The main reasons that QF haven't used the rear stairs are: 1. Wet weather issues (in particular, business types who don't want their suits wet), 2. extra staff required on tarmac for OH&S issues and 3. delaying catering access to the rear galley.
LCC's have much smaller catering requirements and thus don't have a problem with pax disembarking through the rear galley.
The push to achieve higher aircraft utilisation might be the driving force behind why they are re-looking at this. Personally, I don't think it's appropriate for a premium carrier to have pax descending/climbing stairs and be subject to the noise and weather issues on the tarmac.