A story in the Chronicle today examines the confirmed reports of graduate student plagiarism by engineering students at Ohio University. I imagine that if this story had been about a Big 10 history department the fallout would be much worse and probably caught much sooner.
While I don't claim to understand the methodology by which engineers operate, I do know that the historiographical chapter of almost every history dissertation - no matter how boring to read- serves a purpose to place one's argument in context with other scholarship and highlight the originality of one's thesis. Even if an ABD candidate was able to hoodwink his or her advisor and plagiarize from another's work, someone else is eventually going to come along while working on a historiography and discover the damning evidence.
In fact, a few years back when I was teaching an undergraduate historiography course one of my students had gotten two dissertations from ILL to use in her paper and she discovered that one of the author's had plagiarized from the other. It made a great teachable moment. The student did wonder what to do with the information... I told her at least footnote it in her paper and that she could probably cause trouble for the perpetrator if she wanted to pursue it. She didn't, but the experience still provided a great example of the importance of originality and proper attribution in one's work.