What's the Economic Use of a Ph.D.?

I read with great interest a column by Michael Berube on rethinking the role and structure of the humanities PhD.  This is largely an internal discussion about fights in which I have no dog (except that I think that Berube and I are agreed that people shouldn't spend ten years toiling in poverty for a degree that leaves them with little earning capacity or job security when they are done.)  

So I read it mostly the way I read Derek Lowe's blog: with an outsider's interest in an insider's discussion of his own world.