I have seen it personally both ways...
I can tell you from experience that having a rested IRO during landing has been more beneficial than having a rested CA/FO pair.

I have caught so many errors as a rested/attentive IRO than I ever caught because I had 30-45 extra mins rest on break as a flying pilot!

I have had more errors slip through the cracks when a IRO blew off monitoring.

We MUST have rested flight crew no doubt. But at the expense of a watchful eye behind us...No WAY!

Here is an idea...let's let the crew decide and not make the FAA dictate who and when you are tired!