They are effectively worthless if it can be shown they are added automatically to every mail. Privileged communication requires something explicit or genuine intent, i.e. the sender writing 'privileged' at the top. It actually can be quite bad if it's shown that the privileged communication was abused (either in terms of piercing all veils or censure).

My experience is people add these signatures because they see other people doing it and they assume it's a good practice or seemingly professional. It may be some inexperienced lawyers recommend it so they have something to say about email policy. It's not hurting anyone beyond eating up mail quotas, right?