As far as I know, placebos don't actually cure diseases, they just improve the patient's subjective feeling of their well-being. I guess in some cases that's all you can do, or maybe you meant in addition to other treatments. But I'm not sure it's ethical.. you're tricking the patient. The ends justifies the means? Not if you think lying is an inherent evil, which it feels like it is. Also if placebos were routinely used as treatments then word would get out and people would be skeptical that any medicine they get might be a placebo, thus lessening their effect and maybe also their likelihood of taking their medicine.

Though placebos do demonstrate the power of the mind/belief in making people feel better, so maybe there's a legitimate way of tapping into that other than lying to patients, such as instructing them to do visualizations.