I would state that another acceptable situation that arises in which I feel child leashes are acceptable are for very high traffic areas and events in which children can be easily separated from their parents and snatched. Theme parks have been seeing a rising number of cases over the last ten years or so which involved small and younger children becoming separated from their parents and then people attempting to snatch them or in some cases "help them to security to find their parents" and it happens very quickly within moments. A child leash would have prevented the potential snatch and even the panic involved for the momentary loss of sight while moving through a thick crowd.
Besides stuff like that and what Napiform posted already I am against their use. I've never found it acceptable that parents don't want to watch their kids and use them at places like the grocery store, library, and mall. None of which are usually high traffic enough to warrant leashing the children. Lazyness (Child Leashes) != Good Parenting.
Agreed. From the POV of a someone who did have an experience with them, the only time my parents put me into a child harness/backpack (the term leash seems a little harsh for what it was and how little it bothered me) was when we went to Disney World and Astroworld. My parents were carrying stuff I didn't want to carry, and they were trying to ween me off of that habit of wanting to be carried. My parents are great, and I turned out to be pretty well adjusted and social even with that in my childhood, lol.
Besides, I thought the backpack bit was pretty stylin'.
Aside from those situations, though, I don't see the point of them being used in a grocery store, or other less crowded areas. Maybe if you have a rowdy child who knocks over things? I'm personally not convinced by that reason.