Of course, such an observation is nothing new. I've certainly heard comments to this effect before. But Dunn's piece is noteworthy for the care he takes to demonstrate this point with a multitude of examples throughout the Bible and in a great variety of "leadership" positions.
Consider, for example, this reflection on the calling of King David:
“I’m really sorry I let Saul become king,” the Lord said, and sent Samuel out to anoint a new person as king. This time God did the choosing. And once again, he passed over the most qualified candidates to select a boy whose own father didn’t even consider good enough to introduce to Samuel. He was the runt of the litter, a shepherd (which was a job on the level of trash collector today). Yet God thought he would make a good king and instructed Samuel to anoint him with oil and proclaim he would be king one day over all Israel.
And Dunn doesn't shy away from the "failures" of David's tenure as king, having already been so chosen by God:
And God’s man did a good job for the most part. Oh, there was that incident with Bathsheba and the subsequent cover-up that involved knocking off her husband. And there was the census thing that resulted in a plague that wiped out 70,000 people. Hey, everyone has their bad days, right? Yet God still considered David a man after his own heart. But from our viewpoint, David’s heart seems as far away from God’s heart as it can be. What is it that God sees that we don’t?
Now, I do hasten to note that we have other "good" kings in the Old Testament accounts who don't seem to have David's rather serious problems with adultery and murder (to say nothing of that census, which although the Bible is clear enough that it was undeniably disastrous, is somewhat less clear if David did it because God told him to or if Satan did!), but that changes little. The Bible seems to gloss over these problems when it comes to giving it's opinion of how good a king David was. "A man after God's own heart," we're often told. That says it all, doesn't it?
One wonders, given this mountain of evidence that God chooses people without regard to what kinds of qualifications they've received, why we even bother with seminary education, to say nothing of the other kinds of institutions (many explicitly Christian) that exist to equip men and women of faith to do what (we claim?) God calls them to do. This is hardly limited to any particular theological or ideological leaning, either. It's not like "liberal" Christians have such schools while "conservative" ones don't. There are options enough for all. Are we hypocrites?
I don't think so. To acknowledge that God sees beyond human vision to call (and equip!) people to do things for which we may not consider them qualified is hardly the same thing as suggesting that "qualified" people should be barred from doing God's work. Indeed, there is plenty of work for all, and the Bible shows how God uses our experience and training to do God's work, as well. For example, while Dunn is certainly right to point out Paul's ill-suitedness to Christian ministry on the basis of his former life (as Saul) persecuting them, Paul makes more than adequate use of his Pharisaic training to craft writings to the churches of his day that have blessed Christians through all the centuries since then.
Training for God's work remains important. Still, it's probably a good thing that God does something "off the wall" every now and then, if only to remind us that God sees things in ways we can scarcely imagine!