Bose - you can ignore AOPA's scheme as you wish as this is still work in progress but the point is generally valid whether the mentor is an AOPA appointee or just another PPL who reckons he knows what he is doing.
The point is whenever you place yourself in a position of being an instructor or a mentor in its most general context whether or not you are strictly in command there is a good chance the pilot in the left seat has already derogated some responsibility to you - you may not like it, or even be aware of it, but it doesnt make any difference.
I recall being in the middle of Germany with a mate who had done very little touring. When he got lost he was very quick to give me the stick and with a grin - its over to you now. I am sure you have been there.
If there is even a modicum of a chance that the pilot in command is relying on your skills it is well worth doing the job of the commander even if strictly it is not your responsibility.