I'm a local, and normally live my life in sandals, and flips... Except when we go to the park - DH wears vans, or deck shoes or sometimes athletic shoes, for me Skechers, Keds, Blowfish or my athletic shoes.... all light weight.. why you wonder.... well for several reasons, comfort first of all, sandals even the ones created for walking... still in my opinion do not give enough support for all day walking... plus leaving your feet out in the open, you can face sunburn which if you have ever had sunburned feet is not fun and quite painful, this alone could ruin your trip. Then think about all the stroller, scooters, or wheelchairs... zipping about... on Saturday night we were Epcot, we stop to take a restroom break, and this lady leaving the restroom gets on the
scooter, and takes off knocking her grandchild and another woman to the ground, plows across the walk way, and runs into the curb, almost knocking herself off it in the process... while she was plowing across the walkway, she almost ran over several other people, who where bobbing and weaving to stay out of her path... which leads me to traction I personally feel you get better traction in shoes especially when facing afternoon thunder showers, much more stability... and if you have to run for cover due to the weather or scooter you will not run out of them...
I bring several options of foot wear to change it up, as well good socks or footlets... one of my favorite tricks is to bring an extra pair of sock or footlets with me into the parks and then change them when I need to, bring a ziploc or a baggie to put the soiled socks into so your day back doesn't start smelling... a fresh pair of socks is a good way to go... In the hotel room you can use sneaker balls for shoes that might not be smelling so fresh... these little gems work well...