"The purpose of the placebo group is to account for the placebo effect, that is, effects from treatment that do not depend on the treatment itself"
The point of using an attenuated virus is that this is a "challenge model" that is supposed to simulate an exposure to the real virus closely enough that one can judge if the vaccine is likely working, without exposing the patients to the risks of infection by the full-strength virus. CNN did a much better job than Wired in explaining this. http://www.cnn.com/2016/03/16/health/dengue-vaccine-effectiv...
Therefore, the fact that the placebo group had certain levels of various symptoms (according to the article, 80% had a rash, 20% had nausea, etc.) is still important data for other scientists who are trying to judge if they should use the rDEN2Δ30 challenge model in their candidate vaccine trials. It's not intended to provide data on what natural dengue fever is like, which I agree would be unethical and unnecessary.
In a cancer trial you'd typically compare standard of care with the new drug.
Plus, there are some cancers out there with very few working treatments (so no real SoC) - If SoC is not available you would not consider a placebo group anyway.
What I haven't seen is a clinical trial without a control arm.
In terms of the ethical considers it's not unusual to run a trial for a yeat and then switch every patient over to the new drug.