Its a hot debated subject....
I would say the reason is because a small percentage of people who have used it develop an allergic reaction (particularly people who are allergic to plants in the Compositae...or Daisy family...) This has led to its external application to come under harsh scrutiny.... I think the plant would be very beneficial on cuts provided you arent allergic to the plant family. And as always the difference between a poison and a medicine lays in the dosage.
better stated by Paracelsus (considered the father of toxicology)... "All things are poison and nothing is without poison, only the dose permits something not to be poisonous."
You must not use this herb without proper supervision (ie a DR. or if you have been living in switzerland, russia, germany or other countries where this is native and your grandma and her mom and moms mom and so on have been using and making tinctures since the 1500s)
It has very strong bacteriacidal and antimicrobial properties even against salmonella...(which is why it was employed at one time for gunshot wounds)...
I just believe it must be used in proper dilution in order to be safe, however its been used topically for many ailments as well as internally for mouth and throat problems...
Its not for everyone, but hey I would use it for myself... and just sharing the knowledge I have with everyone else
http://www.umm.edu/altmed/articles/arnica-000222.htm
http://www.medicinenet.com/arnica_arnic ... rticle.htm
http://www.drugs.com/npc/arnica.html
I love learning about herbs... good stuff