To the OP,
Before I had kids I would have agreed with you. Once I had kids, I realized the wisdom of using a harness on my child in particularly crowded environments -- most notably WDW and fairs. I have used a harness on DS (now 4) a handful of times in his life and, at those times, been happy I had the harness. Now that DD is walking, we'll be taking the harness with us to WDW in December.
Using a harness is not an excuse to let one's child roam free and wild -- I still hold my child's hand as much as possible. But it is nice to know that I won't lose my daughter if I glance in a window for a second (and it only takes a second). After all, I would be much more distraught if I lost my child than if I lost a dog.
BTW, every time I have used the harness, I have gotten queries from people asking where I purchased it.