John Piper answers that question in three and a half minutes (transcript | audio | video):

My initial response yesterday after reading and watching Piper’s answer was this:

I think I understand where Piper is coming from here, but it seems to me like he devalues his PhD without sufficient warrant. Did the PhD not help him hone his ability to think and communicate clearly and carefully?

I just read Dane Ortlund’s response to Piper’s answer. It’s excellent. (Dane just completed a PhD in New Testament at Wheaton under Doug Moo.)

Update: Just for the record, I certainly don’t think that all pastors should get PhDs. (But that’s not the point of this post. I am questioning whether we should devalue them so much.)