I don't know. I found two Ph.D. students who's work ended up being pretty important for them... and the rest of the world:
http://infolab.stanford.edu/~sergey/ http://infolab.stanford.edu/~page/
http://infolab.stanford.edu/~sergey/ http://infolab.stanford.edu/~page/
If you think I am implying the PhD's do not produce important work, I think you are mistaken.