Totally agree. When I said "legally", I meant '
CASA legally'! I've never seen a reg that says you
can't use a phone as a backup, but then again, the regs are going through much transition at the moment, and CASA's official documentation is certainly lacking when it comes to modern technology usage.
AC 91-17 (replacement for CAAP 233-1) states:
There should be at least one means of access to backup material available to the pilot in command at the time of flight dispatch. This can be either another EFB (it may belong to another member of the crew), or paper charts and documents required for the sector being flown.
Hardly legally binding!