I think we could come up with much more than 2 succesful cases. I think it happens fairly often. Also, the succesful cases, in some ways, outweigh the failures. Two people finding each other vs. some difficulty in the dojo? For me, the cost benefit analysis says that some times, you have to take the risk.
Of course, that's not to condone in any way the serial predators, and it doesn't matter whether they are sensei or sempai.