http://beacons.amsa.gov.au/activation/index.asp said:
When and how to use your beacon
Two-way communications via a mobile/satellite phone or radio is the most effective means of communication when in a distress situation, with some of the reasons outlined below:
Instant confirmation that the call has been received by emergency services;
Ability to communicate position, nature of distress, required assistance;
Emergency services can adequately scale response assets according to required assistance;
Ability to provide updates on the situation and also receive advice until emergency services arrive.
If two-way communications are not available, then a distress beacon should be activated in situations of grave and imminent danger. This equates to when you feel you are facing a life threatening situation. This is a personal decision that is different for everybody.
Personally I think they did the right thing not activating it - they were well prepared & supplied + there was no immediate life threatening situation. They obviously were comfortable they weren't in any grave or imminent danger - just hopelessly bogged & running late.
Good to see a happy ending & some well schooled/prepared explorers although they may add a satellite phone to their kit?