It's more secure than not keeping it semi-secret. If you're waiting for a 100% perfect security solution to be universally supported by every company, I've got

It's more secure than not keeping it semi-secret. If you're waiting for a 100% perfect security solution to be universally supported by every company, I've got bad news for you. SSNs are also terrible security checks and should not be used in that way, but so long as they are, I'll refrain from putting mine in my email signature.