I certainly do not wish to create controversy by disagreeing with esteemed posters, and I fully consider myself to know next to nothing compared to most of the "old timers" here.
My experience though has been to witness my Son progress with each "lesson". Really, we do not mess too much with mechanics, but pitch selection and reading at this point. Mainly still trying to create the muscle memory that will carry him, once he is away from home and beyond. The instructor we use does focus on hitting (for power) but he also holds my Son accountable in the areas of weight gain, flexibility, fielding mechanics and the mental aspects of his game.
And this is the off season, when players separate themselves through hard work.
What happens in the Pro's, do the hitters not work with an instructor that assists them in developing their swing? Did not A-Rod's instructor work with him on his stride? Is this something that only occurs once or twice in a season? Or will an instructor in MLB watch his players during BP and provide input if requested by the player?
Are not the truly great hitters working on being great hitters? Perhaps for some it just happens that way...
When mine was 3 he hit line drives consistently. (wiffle ball and bat back then, lol). He works on his hitting daily, then meets with his instructor weekly, really we have stepped it up the last 3 months (to weekly) because he is entering his Senior Season and would like to have a "break-out" season.
We trust his instructor, in fact we asked him to mentor our guy through the levels of play. He has been teaching baseball for more then 20 years and has worked with quite a few playing MLB, which he himself also played for many years.
I agree that for this to be effective you need to have the right instructor working with the right student. However, it has worked for my Son, and it may work for others as well.
I now realize that this is a "Coaches Forum" and will refrain from posting here further, sorry!