Aren't most of those studies done by medical ppl who have vested interests in prescription drugs? (or done by drug companies themselves!)
Maybe herbs helps keep your immune system healthy, thus helping prevent illness? I've sipped green tea for a long time.. and just now are they "jumping on the band wagon" and selling the goodness of it.
Good point, ((myself)) about before taking anything any herb etc.. ask the pharmacist if it will conflict with something else you're taking.

Herbs can be potent helpers, or hinderers.