The only reason we don't routinely have more than 2 sets of eyes and ears on the flight deck in normal ops is cost, so certain technologies were developed and employed to wind the min crew down to 2.
Granted, the aug crew needs to know what's appropriate and not, though where there is discussion about procedures, performance, other life-threateneing stuff, I'm in the loop - it's my bum too, and just because I'm not the operating captan, dare I say it at this juncture, won't obsolve me from the responsibility of taking any reasonable opportunity to prevent a problem (at best) or having to answer difficult questions as to why I took no notice.
Ask yourself, how would the ensuing investigation view an aug crew's lack of 'oversight' that resulted in a problem? Perhaps we are about to have a definitive answer on that one. Realise one thing though: as the aug crew you are sufficiently disconnected from the operation to have the spare capacity to recognise when things are not right. Seize the opportunity. How you then approach the question of intervention is up to your CRM skills.
I AM on headset for arrival and departure, and have been pleased to be so several times, with incorrectly understood clearances, and other little career-affecting gems cropping up a the end of a long day..
Perhaps some of us here are frightened of being monitored, fearing scrutiny. If so, it's time to question your own operation: if it goes wrong, your life will be nothing but scrutiny. Ask the poor sods imn MEL: at least we can.
Now, back to what happened in MEL...