Don't forget that while the placebo effect positively effects how effective something is, the reverse is true aswell. Someone who put on a band who refused to accept it would work could also essentially ignore the benefit due to their strong belief it wouldn't work and in the opposite way to the placebo effect, essentially force it to fail.
Clinical testing is insanely difficult basically. What if the bands for instance really did give a actual 5% benefit, but the placebo effect meant someone was trying 5% harder than normal just because they believed it would work, giving an overall 10% benefit, while someone who thought it wouldn't work, tried less hard in testing either on purpose or subconciously, making it appear to even negatively effect performance.
Same can be true of pain med and pain med studies, if you're 100% sure a pain pill won't work you can essentially make your mind think you are still in pain even when the actual source of the pain is completely blocked by say morphine.
The mind is one of the most powerful things in determining how well we perform, if a £40 band makes you perform better our of sheer belief and self confidence, its worked. THe question is, if it was only £5, and everyone had one, would the psychological effect be as strong, most likely, not
our minds are a ridiculous mindfield of ridiculous behaviour that often flies in the face of logic and common sense and theres little we can do about it.