Re: "I wonder how many pitchers I've worked with who, despite my wanting them throwing from their most natural arm slot, were in fact throwing from an altered arm slot thank to previous coaches. I think this is a dilemma that those of us who favor NOT messing with a pitcher's arm slot are faced with constantly. It certainly adds legitimacy to DM's perspective. "
----This is a very good point, and it often seems to raise the question: How does one determine a pitcher's "natural" arm-slot?
I'm not sure this is a perfect answer, but House taught it to me awhile back and I feel comfortable that it makes at least as much sense as anything else I've heard on the subject.
The advice was this: Without telling the pitcher what you are actually up to, take him over to a corner of the outfield and tell him you are going to roll some grounders to him from various distances. He must field the ground balls and throw them back to you. Tell him this is just some warm-ups for PFP or any damn thing that is unrelated to pitching itself.
Watch his mechanics, his balance, and his arm-angle. If you observe that he is most balanced, comfortable, and accurate throwing from 3/4, over the top, sidearm, or whatever, in this unspoken evaluation...then, while his guard is down and he is relaxed and not thinking about pitching the ball, he is actually telling you what his most "natural" pitching arm-angle is.
Undoubtedly, many pitchers can be successfully re-trained to throw from any new arm-slot that better fufills a coach's vision of "the perfect arm-slot". Nevertheless, the fact appears to be that every humanly accessible arm-slot has been well-represented among MLB pitchers, so a cookie-cutter approach to arm-angle may not be so good, and one coach's "vision" may be the end of some kids' "career" if that vision happens to be misguided.