The frequency boundaries are there for ATC workload and management purposes.
The boundaries are FIA boundaries, an area within which an air traffic service is provided by an ATC responsible for the area:
Flight Information Area (FIA): An airspace of defined dimensions, excluding controlled airspace, within which flight information and SAR alerting services are provided by an ATS unit.
As
triangulation says, there really isn't a conflict between an FIA and a CTAF, the former being an area within which an ATS is provided by the ATC responsible for the area, and the latter by definition being a
frequency to be used when at or in the vicinity of an AD.
They've worked successfully together for donkey's years (except for 3 months) until in 2014 (?) CASA meddled in AIP with the frequency to be used for unmarked strips and tried to define CTAFs as having specific lateral and vertical limits.
So personally I don't think there are reasons for change, if CASA were to roll back the AIP changes in some form.
However I can concede to MULTICOM existing below 3000 or 5000FT, recognising that it would (a) add another frequency for climbing and descending aircraft to monitor and broadcast on and (b) result other issues e.g. for aircraft experiencing an emergency, potential frequency congestion due to comms from aircraft being heard over a wide area etc.