I can't speak for everyone in the field, but it's the anachronism for me. The Middle Ages thought in anachronism, and were more open to class mobility than it appears. So I guess the idealism does play something of a role.
The story I'm thinking of is the Vulgate Lancelot, a long prose Old French version. Unicron, you're right to suggest such a stunt was dangerous: that's part of the point of the episode, not just to humiliate Lancelot but also to endanger him. It is, of course, Guinevere who orders him to do it.