The Cat and Fiddle route is mainly dangerous because a lot of bikers use it for speed runs. It's not uncommon for a bike to appear around a corner, on the wrong side of the road, doing in the region of 100mph - hence the high number of fatalities around there.
There is a big safety campaign on at the moment to encourage sports bikers to drive in slightly less suicidal manner - I've noticed a few signs around Oxford and Banbury, where I've been working this week, but where I live, much closer to London, there aren't really the roads for that kind of riding.... which is a bit of a relief, to be honest. As a car (or in my work, van) driver, there are few things more alarming than a bike just suddenly appearing in your mirror or right in front of you, going far to fast for you to avoid it if things go wrong. Don't get me wrong, I like to drive fast in my car, but I always feel I'm in control and I do observe speed limits closely in traffic and in built up areas. Driving for work, I'm even more careful because I have a relatively small, but nonetheless bulky and heavy van to drive around. Unfortunately, a small group of bikers do not have the same respect for other drivers