These are of course the very small minority to the general rule.
There are also cases of it working the other-way: PhDs that have made a lifelong contribution to their field without an academic post. Paul Erdős comes to mind... the most prolific mathematician of all time (by number of published papers) was a vagabond. http://en.wikipedia.org/wiki/Paul_Erdos
(Interesting fact: both Wittgenstein and Erdős were disciples of the thought of Frank P. Ramsey: http://en.wikipedia.org/wiki/Frank_P._Ramsey I am finding it hard to discover which degrees, if any, he had.)