parazapper
You may be right about the tendonitis thing, but battery tapping does work for those who can not afford better.
As for scientific results, who is going to pay for those? Consider that a study for killing bacteria on the skin ( about the cheapest study possible ), can cost about $100,000.00 US, if done in a 3rd world country, and much more in the US, Canada, etc. A study for zapping malaria would cost $$$$Millions.
As for the value of a zapper, even a cheap zapper can cost 6 months average income in some African nations, yet plenty are sold there. They are purchased mostly for protection against Malaria, filaria, HIV, etc.They are being bought because those that have, do have an advantage over those that do not have.