Part of it depends on the size of the group. Group of five friends, its a fine rule. Everyone knows their turn is going to come up. He already said someone usually picks up the tab anyway. This sort of thing works as long as you have the same group of riders and the overall cost is relatively low. I'd have no problem with the 2-3 people I usually ride with. At least among my friends, I could actually see someone volunteering out of turn (or intentionally riding over glass), just because that's how they are.

20+ people group ride, you've got random people showing up, unfairness, etc.