I have a problem with this notion of a "light responsibility" position. I think any position can be "light responsibility" or heavy responsibility, depending on what you (and the scout who holds them) make of it. 
I have served both as a PL and a Troop scribe, and could honestly say I did more work as a Scribe than I did as PL.   
This was probably do to the fact that our troop operated on more of a "troop method" (if such a thing can be said to exist), than it did use the patrol method.  PL's were expected to come to PLC meetings, and occasionally plan a game for the meeting or teach a scout-skill.  As scribe, I too came to PLC meetings, and actually took minutes at them, and then typed up and sent out copies of those minutes to each member of the PLC.  I also (tried) to organize the taking of attendance at troop meetings, and I am sure that their are some troop scribes who are responsible for some aspect of money collection.