Wired clients do not need a security protocol per se.
But an open network is *completely* open to the extent that even files that are not currently shared can soon be accessed by anyone with a bit of knowledge. So, by connecting to a wifi network, one can access any and all clients on that network, even the wired ones. Just like you yourself can.
Of course, this is not commonly of much interest to your average wardriver, who wants to use a free and anonymous internet connection, but do you really want them to have the opportunity to snoop?