Dr. John Piper offers his take on this question over here. My experience was different than Piper’s. My doctoral research has definitely enriched my ministry. A big reason for that was that I was able to study with an institution that shared my values and confessional commitments. I highly recommend Reformation International Theological Seminary (RITS) to anyone looking for a doctoral program that’s both theologically rigorous and confessionally oriented. I recently became aware of another local Reformed pastor (of a NAPARC church) who is working on a doctorate with RITS. Some men you may have heard of with their doctoral degree from RITS: David Murray (professor at Puritan Reformed Seminary) and Robert Grossmann (formerly of Mid-America, now with Heidelberg Theological Seminary).