Hi guyz, I saw this topic but just now I had time to actually withstand the 11 pages of nonsense. So let's get to it.
I think the entire problem lies in that the answer to the thread's question is a question, "For what?" And also, of course, what the definition of "training" can be. Training can mean anything you do that relates to composition. Going to a concert is part of "training," for example, as it exposes you to the medium and gives you a chance to experience things first hand. Learning technical knowledge is also a type of training, as is learning to talk about your music and/or listening to others do it.
For the question to be meaningful it's necessary to narrow down exactly what the person wants to do, before any kind of method can be looked into. For example, if someone wants to write symphonies in style of X composer, it's advisable then that the person seek out these compositions and study them (how is a different question altogether.) However it's only because there's already a set goal with defined parameters and stylistic guidelines that this is necessary. If the objective was something more abstract like "Write something that sounds good," as is more typical, it's possible that direct experimentation rather than specific technique study is much more valuable as a type of training.
As this is a pedagogic question, it should be clear that different people require different methods and a single didactic approach can't hope to fit even a small minority of people, specially when it comes to something as vague as "teaching art." For this reason, then, let's look at the first post and formulate an answer that would fit the more exact parameters defined within the overall question:
It's impossible to give a proper answer when it concerns potential, as it depends on too many factors that are unpredictable. Also, "formal training" is indistinguishable from simply knowing what you want to do and finding a method of accomplishing it, as I stated previously different objectives can be accomplished in different ways and it depends on the person what kind of approach works best.
The importance of so-called "Formal" training here is the aspect of guidance, such as what I'm doing now, in terms of what methods the person may find useful in reaching their artistic objectives. It should be clear such guidance doesn't necessarily have to come from a teacher figure or an academic institution, it could be even a random suggestion on an internet forum (or reading history books, like he did when he cited Elgar's example.)
As a closing thought, I think it goes without saying that none of this should concern itself with "quality" of whatever it is the person is trying to produce, but simply with the method by which they can reach a result they're happy with regardless of what it is.