First to clarify what I am not arguing:
I'm not arguing that a placebo pill has an inherent effectiveness in and of itself, nor am I arguing that placebos should or should not be used as a baseline to measure other drugs against. I'm not arguing that the placebo effect has been shown to remedy serious illness.
What I am arguing against is the stance the belief placebos is inherently harmful, and by the nature of its irrationality, bad in all cases.
Positive thinking if defined as saying to yourself "I'm going to beat this illness...", is not the same as a placebo. The idea that you will in the future beat the illness is much different than thinking that you are presently beating the illness. This a distinction worth noting. If you will do something, then it means are not doing it now. Thus your symptoms are not improving. Maybe in the future they will. Now, if the positive thinker affirmed and believed "I am beating this illness through the power of my positive thinking," that would be the same as a placebo, would have the placebo effect, and would be labelled the same way by some as "lying to yourself".
I acknowledge that if a medicine, scientifically shown to be effective in a double-blind study, is available for a treatment of course it should be used. However, if there is no cure for the treatment, I argue that it is immoral to try to take away a person's belief in the placebo. You would rather that person not use the irrational power of their own mind to improve their condition simply because it is irrational. (It seems to be a recurring theme throughout this forum that belief without evidence is irrational and must be abolished.) Because belief is required, the placebo effect cannot be rationalized. By being completely rational, you cannot have the placebo effect unless it accompanies something that you believe is an effective treatment.
If you take away the belief of someone, you give them little else except what? Positive thinking for something to happen in the future? It is better instead to help someone keep looking for a scientifically proven effective treatment while not discouraging their use of the placebo. That is the best of both worlds.