Back markers – in my opinion it is entirely up to a particular leader if one should be appointed. Circumstances vary. Leading styles vary (I myself prefer to be within the group rather than ahead).
If you are doing a C walk with say 24 walkers it is probable that the group will stay reasonably together. And in any case at such a pace the leader can probably drop back three or four times in the course of a walk to talk to the people at the back to ensure they are okay. Our programme states that everyone on a walk should help to keep a group together and this includes notifying the leader if someone is struggling at the back. In such a case I personally don’t think a Back Marker is needed.
On the other hand if you are doing an A walk with 24 walkers it is likely the group will spread out much more as individual walkers pace will vary considerably. On such a walk it is unlikely a leader would be able to drift to the back and then get back up to the front in order to give directions. While each individual walker should still help the leader by pointing out someone is struggling it is obviously much more important that a Back Marker be appointed.
There is of course the question of who is suitable to be a Back Marker. I wouldn’t want someone who hasn’t some experience of leading. Preferably of course the person should be familiar with the route – but while most people like being up at the front not many like being at the back – especially with the obligations of being a Back Marker.
But as I said above IT IS UP TO THE INDIVIDUAL LEADER – no one else.