Colin Murray said:
It's my experience that people who have the strongest opinions about child-rearing have the least experience in the matter. It's folly to think of children as tiny adults, or that they're capable of being demeaned the way adults would be. The kid might not like a harness but if it's keeping them from being hit by a car you really can't argue with the results.
Alternatively you could, I dunno, watch the kid? While I haven't had kids, I've taken care of my neighbours 5 year old at various points in the last 5 years, their 2 year old for the last two and my sister's daughters from a range of ages so I have at least a little experience in this and I'm pretty anti-harness after a while.
Not because I don't think it's possible they'd wander into traffic but generally if we're near a road I'll make them hold my hand or carry them or any number of other reasonable options that a)mean I'm paying attention and b)mean they can't do it anyway (because you can still have a toddler wander in front of a car on a leash they're just strapped to someone while they do it, it's really not a preventative measure in that regard). So personally, I don't like the idea. On the other hand -
Blablahb said:
But the chance of a child accidentally running off somewhere a car hits them is tiny. It's probably along the same lines as the chance of getting struck by lightning.
And even if, my parents always taught me to never run into the street without looking. That seems preferable an option to treating your child like some sort of animal.
So to answer the OP post, there's never a justified reason for putting your child on a leash.
The chances of a toddler running in front of a car when next to a road are probably pretty close to 1. Toddlers don't understand the concepts of cars, danger or mortality so are probably more concerned with something like "Look! Red! Approach with face and lick!". The age range we're considering most often seem to have the self preservation extincts of a brain-damaged squirrel so expecting them to use logic is unfeasible (there's only so often you can say, 'stop trying to grab the bacon as it's cooking' before you realise they understand but flat out don't care). Because I think we all agree 5-6 year olds shouldn't be on a leash because what the crap parents, if they can spell their own name and own a goldfish they don't need to be leashed.
There IS a justified reason to have kids on a leash, that being you're safety conscious but also want your children to run around and exercise. The alternative is usually putting them in a pram (or whatever the american word for that is, baby carriage?) so they're safe but costs them exercise or do what I do and use your long legs and spatial awareness to keep them watched and safe or what not. Which is fine for a young 20 something, probably less so a 60 year old with back pain or a mid forties obese individual who has somehow inexplicably procreated and now has to watch an 18 month old baby who is significantly faster than them.
So in summation, I don't support the use of leashes but I understand the practicality and decision to use them.
tl;dr, I don't like leashes, some people might use them and while I disagree in certain circumstances it makes sense. Feeding your kid treats for behaving and letting them poop in the park is kind of a 100% unacceptable rating.