When those leash things 1st become popular I thought "omg, how horrible!", but I don't think that way anymore. I think it does give an active toddler more freedom than being strapped down in a stroller the whole time (I"m not saying a stroller is bad - but some kids like to move more.)
A shorter lead definitely seems like a safer option than a long one that can clothesline others.
I don't have kids, and I own my own faults - being judgemental is one of them. So here it is - parents: please HYDRATE your children. If your child is thirsty, get them a drink. I'm not saying give in to your child's every whim & buy them the fancy drink or the giant soda or whatever. But, if you don't bring your own water into the park, and don't like the taste of the water from the water fountains (it is pretty gross), then be prepared to shell out the, what, 3 bucks, a few times a day for your kid. Please budget for this.
Lately it seems I see a lot of hot, miserable looking kids when I'm in the parks, and I wonder why nobody bought the 20 oz Disani bottle. It's what, 3 or 4 dollars. Yes, it's expensive. But, you can buy one, and then refill from the water fountain if you can stand the taste (or get a free ice water from the QS places or stands that have fountain drinks.) Budgeting for tix but not to keep kids safely hydrated is a strange choice in my view.