I haven't had access to FTA for free in over 10 years. I don't have an aerial and rabbit ears don't work at my place, so I've been paying $10 a month for a Foxtel box just to access it. Once they're all live streaming I can send the box back.
Data allowance isn't an issue in my household. Telstra increased my cap from 200GB to 500GB for free and it's a gut bucket load more than I could ever use.
Other people just find it inconvenient to have to change the settings on their TV and Amplifier when switching between ATV and FTA. That's only become an issue for me now that I use HDMI CEC with the ATV. Switching to Foxtel screws it up.
If you ask someone else that's using it I'm sure they'll have different reasons. Maybe they're expats and love being able to get Australian FTA again, maybe they just like their entire TV experience in one place, maybe they have 3 TVs but only one antenna socket....heaps of reasons.