Here's the rub. A lot of people show up at the doctor for things which will take n days to go away - with or without treatment. The common cold, for example. They won't accept NOT getting any prescription and will hop from doctor to doctor until they get one.
Now the best thing would be educating the public about this issue. This is very, very hard to do. Barring that, it is actually better for the patients and cheaper to just prescribe placebos - they DO work in this case! (up to the placebo effect, as any other medicine would).
Unfortunately there is another issue involved: Most placebos (at least in Germany) are homeopatic. This lends credibility to the whole homeopatic industry, and THEY are nothing but quacks. And THAT is a bad thing.
So - either way you lose.