John Bunyan - SOME grammar school.
John Owen, scholar of scholars, said (paraphrasing) he'd trade all of his academic career and acumen, if he could preach like that ignorant tinker (John Bunyan).
Yes, but it is exceptions that
test the rule.
Why would we be content, as a rule, to accept untrained ministers? We wouldn't accept physicians without formal training, no matter how pious they were, would we? Would we be happy with supreme court justices -- e.g., Samuel Alito -- without a law degree, but great piety? Why would we operate with such a standard for ministers?
Implicit in the argument that the only thing that is necessary is piety is the assumption that the ministry does not require learning or only very narrow learning or that the ministry is not as important as other offices (e.g., the law or medicine etc).
Confessional Reformed folk have never accepted the pietist argument that all that is truly necessary for ministry is regeneration, piety, and zeal. In fact, most of the early pietists were highly educated! Once, however, this standard of is accepted subsequent generations decline in their knowledge and ability to function.
The ministry is incredibly demanding. Nearly EVERY pastor I´ve ever met wishes a) he had paid attention in seminary; b) had more time for study now; c) could go back for MORE training.
Pastors are generalists. They must answer questions in all fields of theology and biblical study and practica. More than that, they must make deductions and applications in difficult circumstances, sometimes with no preparation. There must be a well of training and learning from which to draw. If there is no such well, the answers will likely be just as shallow.
If I may speak frankly, those who don´t want such things, in my experience, are foolish and arrogant and not very useful arrows in the quiver of the Lord. Yes, the Lord can still use donkeys, but why test Providence?
As for local churches doing the training, as has been pointed out in this discussion many times, most local pastors are not expert in all the areas needed. Most local pastors KNOW this and would not present themselves as sufficiently expert in all the necessary areas.
There was a time when a very bright and intelligent person could know most of what could be known. That time has long passed. With the explosion of publishing (which is only growing exponentially with the advent of the computer and the web) has come an explosion of knowledge.
I teach in three fields (Historical theology, Systematics, and church history) which is very unusual. I can barely keep up with them, even though they are related.
Think of trying to keep up with ALL the fields (7 loci of systematics; Patristics (500 years of history in 2 primary languages and a huge body of secondary literature in 3-4 languages), medieval history (1000 years of history in Latin and a huge body of secondary literature in 3-4 languages), Reformation (Luther's German and Latin works fill 50+ volumes alone, the Corpus Reformatorum is in German and Latin and fills 109 volumes; the secondary lit is in 3 languages and my
select bibliography
http://public.csusm.edu/guests/rsclark/Reformation_Bibliography.htm runs to 22 single spaced pages), post-Reformation, Modern; OT (with dozens of sub-disciplines), NT (with dozens of sub-disciplines) and with all the necessary languages (Hebrew, (and 4-5 other ancient near eastern languages) Aramaic, Greek, Latin, German, French, etc. Then there's practical/pastoral/ecclesiastical theology.
I do this full-time (and pastor part-time) and I can barely keep up. There isn't a local pastor or even a team of local pastors who are engaged in the work of the ministry full-time, writing sermons, making visits, sitting in session/consistory meetings etc who has the time to keep up fully with any two of these 50-60 areas of expertise.
So, as I've been arguing for years, we have two choices: a learned ministry taught by specialists in seminaries or an ignorant ministry taught by other, well-meaning, but slightly less ignorant people.
In the latter case, those so trained will train the next generation etc. If we choose this path our ministry will gradually spin its way into a dark oblivion.
rsc
)
Like I said, the question should not be "should we require our pastors to have a formal education" - but "where best for them to get that formal education". 
Can it be done through a virtual university? What seminaries can we trust to do the job? We all hear about the young eager Christian going to seminary just to have all his beliefs torn away by liberal and/or neo-orthodox professors. Picking the wrong place to get a seminary degree is worse than not getting one at all. 
But there is no doubt in my mind that a formal education is one of the best means of equipping pastors and elders with some of the tools they will need for preaching, teaching.
