I tried to talk about a seminary directly, but we need to step back and look at the entire issue before we can approach that subject with any hope of communication. So, lets begin with the process of analysis, reducing the large issue into its elements.
First, let's look at what is beyond our scope: being clergy. One's legal Status as a member of the clergy, is determined by the relevant laws. One's Ecclesiastical Status as member of the clergy, is determined by the Ecclesiastical body. So, to recap, things to do with whether one is or is not clergy, in any sense, are matters of Church and Law. In the US the Universal Life Church combines these into a one stop ordination shop, so it isn't exactly a difficult status to obtain if that is all one wants.
Seminaries exist to train and prepare people to practice ministry, and provide related education. They do not grant the status of being clergy. Law Schools exist to train and prepare people to practice Law, and provide related education. They do not grant the status of being a member of the bar. Sure, most of those attending Law School intend to become practicing lawyers, but only a percentage of them will. Most of those entering seminaries intend to become clergy, but only a percentage of them will. And there are those who don't want any change of status, they just want to learn and grow.
Not all denominations that use seminaries, require them. If there are cases where graduating ensures ordination and the status of clergy, I am unaware of them. So, we really need to disentangle these two concepts: preparation and ecclesiastical/legal status.
As far as preparation for ministry is concerned, it is not a simple one-dimensional thing. We can discern different elements, broadly speaking: education, formation, and personal development/preparation.
In terms of education, does it matter greatly who is involved in the education and how? If a bishop does not grade all of the papers involved in a candidates education, has she lost the ability to discern whether the candidate is properly prepared? Does a member of one's particular denomination have to give out reading assignments for the reading to be valuable? I get the impression that this isn't much further down the path of some notions, and we should acknowledge them as being ridiculous.
In fact, the opposite is more likely, one is more likely to get an adequate preparation in terms of education from an institution set up for that purpose. And unless one is afraid of being “contaminated” by ideas from beyond one's denomination, the only issue becomes the quality and adequacy of such an educational program. The Gnostic bottom line: does it work?
Education is a part of formation as clergy, and an educational program involves aspects of formation. However, formation as clergy is of a much larger scope, as it involves the granting of that status as we have seen.
Not all aspects of preparation for ministry can be addressed through education. In fact, only education can. However, since educational programs do not confer any status upon anyone in regards to being clergy, why be afraid of education?