Naked Security tells us about a survey in which 77% of respondents expressed no concern for security when using public Wi-Fi. See Most people think public Wi-Fi is safe. Seriously?. It was the British Office of Communications study, but there's no reason to think Americans would be any different. (See page 322 of the report; 326 in PDF.)
Naked Security says:
It's a grievous error to trust Wi-Fi to the degree that these numbers reflect.
What's the harm? Well, companies, people and governments could be spying on you.
They provide some recommendations:
Don't automatically join a network based on its name.
Turn off Wi-Fi and Bluetooth.
Use a VPN.
Turn off geo-tagging for those services like Facebook, Twitter and Instagram that use it.
Do all of those. Also wear a belt and suspenders.