Bruce Levine, one of whose books I reviewed some time ago, has an article on the Greanville Post which asks "Why has the American public not heard psychiatrists in positions of influence on the mass media debunking the chemical imbalance theory?" As Levine says, and as I have previously discussed several times (eg. in an BJPsych eletter), the reason is that the theory is used to persuade patients to take their medication. It may make it easier for patients to accept their depression and take their medication if they believe they have a chemical imbalance in the brain.
Actually, I do find that patients are generally able to understand that the "chemical imbalance theory" is only a theory. In fact the evidence is against it. What they find more difficult to appreciate is why they are told that the theory has been proven, when this is clearly not the case.
Big Pharma is commonly blamed for encouraging the chemical imbalance theory. Actually, drug companies sometimes only say that it is a commonly believed theory, maybe implying but stopping short of indicating that they believe it. The theory wouldn't have survived if it wasn't for psychiatrists' complicity with it. Bruce is right to direct his criticism at psychiatrists themselves.