All it takes is one mistake... You don't regrow fingers. Perhaps it's too far in the past for many to remember, but there was a widely talked about incident in 2008. I can't find the thread dedicated to it (I'm pretty sure there was one), But it's discussed here:
http://www.chiefdelphi.com/forums/sh...&postcount=125
The end result was that someone (I believe it was a college aged mentor) lost part of his finger when it got caught between a chain and sprocket.
You can't just evaluate safety based on likelihood of someone being an idiot. It's a combination of likelihood, random mistakes, and severity of the result. Losing part of your finger is a very severe result, and we try to treat it as such when asking people to put guards on their robot.