I had the same opinion about them when I had my oldest son, who was pretty close to being a perfect baby/toddler.
I would not have thought about using one....then 6 years later came my youngest, red head....who decided at 8 months to not just walk but walk very fast. I didn't use one on him either...but should have. I think they are a great invention! The argument could be made the same with strapping your child into a stroller...How many times have you strapped your kid in because you were in a hurry and didn't want to take the time out to walk at their pace? Or were shopping and strapped them in so you didn't have to wonder if they were roaming off...same thing essentially. I have seen many toddlers crying because they just wanted to get out of the stroller and be able to explore, which is natural for children...This is where the "leash" comes in handy. I see nothing wrong with it at all.