Well, except the backbone has no alternatives, and the whole point of something like Tor is to mitigate one's lack of trust in the people running the backbone. If Tor is insecure, all you're gaining by using it is letting a bunch of three-letter agencies know that your traffic is potentially more interesting than the average.
I think the idea is, yeah, we can have untrustworthy exit nodes, but that's already anticipated as a potential threat so everyone should know better and use https as much as possible, among other means of encrypting exit data. Doesn't Tor at least try to encrypt even exit data as much as it can? Like the browser bundle even comes with https everywhere.
