The use of placebo controls in experiments is not required to make a valid experiment. If you tell the research subjects that there are, say, 4 treatment regimens which the doctors have roughly equal expectation to work, and they're randomly assigned to them, then there's a lot less interference of the sort described above, and the doctors can tell the truth, and the experiment still yields useful information.
The problem of published research findings not being reliable has been discussed here before.
One problem with RCTs that has received little attention is that, due to informed consent laws and ethical considerations, subjects are aware that they might be receiving sham therapy. This differs from the environment outside of the research setting, where people are confident that whatever their doctor prescribes is what they will get from their pharmacist. I can imagine many ways in which subjects' uncertainty about treatment assignment could affect outcomes (adherence is one possible mechanism). I wrote a short paper about this, focusing out what we would ideally estimate if we could lie to subjects, versus what we actually can estimate in RCTs (link). Here is the abstract:
Any thoughts on this? Is this a trivial technical issue or something worth addressing?