The placebo effect is more than just a positive mental attitude. There has been work showing physological effects from placebo treatments specifically dopamine production in patients with Parkinsons.
Ironically you really can treat people with placebos.
http://www.pbs.org/saf/1307/segments/1307-3.ht m
A lot of alternative medecines work on this principle (whether or not they think they do) which is why they tend to fail in double-blind tests and why they are most effective in treating conditions of chronic pain.
So you'd want to be careful about dismissing things as "just" the placebo effect.
There have also been sucessful double blind tests with some herbs. I think there's about 7. I can't remember them all but garlic was definately one and surprisingly (given it's popularity) evening primrose was not.
Just because people used a treatment for thousands of years does not make it effective. Often treatment would be based on a similarity. So a frog would be placed in the mouth to cure a sore throat - the logic being because the frog croaked! - funny but similar logic applied to herbalism Walnuts to treat headache - walnuts look like brains of course.
On the other hand willow bark was used and that contained asprin! It gets real tough sorting out real effects from placebo in herbalism