This video has been making the rounds:
I’m with Piper here (surprisingly) for the simple fact that I think if you’re already pastoring, assuming God has called you to pastor, then you’re doing what you should be doing. I don’t think a PhD is necessarily going to make anyone a better pastor even if it will put them in a position to know more stuff or to think more critically. The best pastors I’ve ever met have all been without formal education. This isn’t to say that they weren’t competent handlers of the Bible, they were, but something they have that formal education can’t give is a heart for God’s people. Now I’m not saying that you shouldn’t scratch and itch to do a PhD if you have it. If it’s something you want to do then do it; just make sure you count the cost first.