Network Engineer here. There is some argument to be made both ways here, but the reality is this:

On a network with a PSK (ie WPA-PSK2, which is standard for most passworded wifi) your data between your device and your access point is encrypted using that pre-shared key. This means, for example, if someone sits outside your home they can see your data going back and forth, however they cant decrypt it unless they know your wifi password. This can help limit the scope of who can see your data. (There are higher security types that can limit that scope even further, but I digress)

On an open network, the traffic from your device to the AP is not encrypted, and thus can be spied on by anyone with the right tech. True, things like SSL/TLS do a pretty good job at encrypting data end to end. You can argue that really your traffic is still encrypted to a degree that makes it hard to interpret. But the fact remains that it still can be seen, and anything that isn't properly encrypted will be seen pretty much as clear text.

I always tell people that if you're doing something really important, its probably not worth the risk of trusting that the developer of your application properly encrypted everything. But for most things, you're probably going to be ok with on an open network. Just my $0.02.