The problem is not "eventing barefoot" ever, I'm sure on a nice day that any horse that is accustom to being barefoot can event barefoot, even over rocks. It's those days when it's raining cats and dogs and the thick grass that makes up the cross country field is drenched and turning into a form of slick mud/grass that a hoot can't dig into properly. When your horse is barefoot your only option is to scratch. It's dangerous to ride a horse without studs in conditions like that. They can't dig in and get traction because their hooves won't get deep enough in.
Now when the horse is rocking studs that give them up to an extra inch spike down into the ground to grab at the solid soil beneath the mud and slick grass, they can still ride the course and the possible $600 dollars they spent on their weekend won't be lost.
Granted, I've seen all kinds of horses slip out on courses, studded, barefoot, and just plain shod. It's horrifying, but it seems to be more likely with horses without studs (in general, barefoot and plain shod). I'd never ride my pony out at a competitive novice level course without studs (even just little road studs). I also think it's stupid to be competing above there without studs in, as long as your farrier knows how to shoe a horse properly, shoes aren't going to hurt, and do provide some extra traction and impact support. And you can't drill and tap a barefoot horse.