If you have a 'number of members' based charging scheme, then members become a cost, that someone has to pay.
In Mark's original scheme, this would fall solely on group owners. Some will be happy with this, some will not.
The advantage of Samuel's proposal is that it permits group owners who are happy to pay to do so; and provides a means for those who are not to have groups in which the members pick up the cost.
As such - giving group owners the choice - it's the better option.