UNPOPULAR OPINION ALERT
I'm one of those people who's very, VERY against the use of harnesses. That being said, I am MORE against bystanders being rude by making disapproving faces at or comments to other people. How dare they!

We're all entitled to our own opinions, but we shouldn't feel entitled to put our own opinions in other people's faces. OK, OK, I know I'm doing that here, but this is a public forum designed to share opinions.
So like I said, I hate the very idea of harnesses, but you shouldn't seriously care for one second how I feel! They're YOUR kids. It's YOUR family. YOU make the rules. YOU make the decisions. You have to do what YOU feel is best for YOUR family.
Use the harness, and if you see disapproving looks from other people, go forth in confidence that you are doing what you feel is best for your family, and those other people are just being rude rude rude.
For those with flames in hand, I have a fairly strong resume. I'm a Dad of a 6 year old and a 3.5 year old. Remember, they used to be younger, I didn't just adopt them at this age. I've been all over with them, all around busy and crowded and dangerous touristy places (WDW, Manhattan many times, downtown Philadelphia many times, Baltimore Inner Harbor twice, Ocean City NJ boardwalk, and we live 4 blocks from the Atlantic City boardwalk). Due to my wife's work responsibilities, I've often taken the kids to these places by myself, without my wife or any other adult. They're active, they RUN, they SPRINT, they don't listen (just like most young ones), and my wife and I are often EXHAUSTED after having them out in public (well, more 2 years ago than now, but you get the idea). Since we don't see any other acceptable options for US, it's no big deal, it's just another day on the job.
AGAIN, bottom line, please don't care if anyone disapproves or not. It's not their business, it's not their worry or concern. If they don't like what you're doing, well they can just go to H ....um ... UNIVERSAL STUDIOS!