I know nothing about long term statistics on ministry dropouts. Of those in my year of college who did MTS (Ministry Training Strategy, a 2 year training programme you do before theological study) at UNSW, and the ones immediately before me and after me, 100% of us are still in ministry, or have returned to ministry after some time in the wilderness.
Now pride comes before a fall, and who knows what failures lie ahead? Or indeed, successes, as we see those who we thought were 'dropouts' from ministry—and sometimes possibly even Christianity—returning to the faith (if indeed they ever left it) and being used by God in all sorts of wonderful ways.
I wouldn't want to sheet home ministry success or failure to a training programme, whether MTS or other, as all of it, every last bit of good that happens, is a work of God's Holy Spirit.
But 2 years doing MTS, as well as being training for ministry, really is ministry. By the end of it, you will have a very good idea of what it involves, along with the opinion of a broad cross-section of people, some of whom are worthy of respect. It's pretty useful, I say, to have that experience before you fork out the dollars and the four years necessary to obtain a decent theological education.
Yes, I recognize that is a pragmatic argument, but I've always been a pragmatist.