The chixulub comparison is quite interesting, it does make you wonder what would happen if you could have your 100 impactors of that size delivered in a short period of time, what effect would that much energy have on the core of the planet (minimal I suppose) and how long would it take for the atmosphere to settle dwon afterwards. Of course the Martian conservation movement would be up in arms.
That's exactly the point though, placebos do work, hence the placebo effect! Furthermore a variety of placebos seems more useful than a simple sugar pill, though the jury is out on the differences between their usefulness.
For the majority of ailments that people visit a doctor for a placebo would be the best treatment option, no-one is saying try and treat cancer with them. If you are being prescribed penicillin or anti-depressants or even mild painkillers you're probably better having nothing at all.
Again, I'm not saying that is better than most drugs, because after all that's how drugs are tested for effectiveness - they must be better than placebos (which are usually also effective!). However in many cases rather than increasing antibiotic resistance or risking addiction issues you'd probably be better having some quack waving a crystal over you.
Most speed limits are intended to be upper limits road and situation allowing, it's actually quite rare to see a minimum speed limit marked. This assumption that you have to do that speed exactly regardless is more likely to make you a hazard.