They're even worse than useless, because the devices paired to those hidden SSIDs have to probe for them explicitly. So instead of having the base station broadcast its SSID in one spot, you instead have clients broadcasting that SSID everywhere they go, disclosing not only the SSID itself but also their association to it. I wish OSes would go a step further and deanonymize hidden SSIDs, just to drive home to everyone that this feature sucks and doesn't work.