I'm getting grief in trying to post my response to the My Boys' Teacher's comment in the comment box. I'll try here, cross my fingers and hope it works!
My mind is going in all kinds of directions on this one.
First thought: Games are games and sports are sports. You need to know how to play if you want to be able to play it. I would expect showing kids how to hold a bat to be a part of the game. But I'm not sure I've ever heard of a Montessori-style sport. ;) I don't really see it any different from playing board games--you need to know what to do. Holding the bat is a basic. If the child is just wanting to hit balls, then maybe a Montessori-style presentation here and there to show the proper hold would work. When you are looking at a game situation... We're not talking general educational principles. We're talking about being part of a team and competition. It's a whole different... um... ball game. (Excuse the pun.)
Second thought: With kids just learning baseball, a lot of Montessori-style presentations would be good for learning how to hold the bat, put yourself in proper stance, etc. Doing it OUTSIDE of the game time as part of preparation rather than correcting them during the game would probably also be better--and definitely in line with Montessori.
Third thought: The child ought to have the choice to play baseball or not and ought to understand what it means to play baseball. If the child is choosing to play baseball, then he is choosing to learn how to play in a specific way, which involves not only rules of the game, but how to hold the bat. Baseball is not Montessori and I don't think one could run a team truly Montessori-style.
Fourth: Playing around on your piano at home is playing around on your piano at home; wanting to be a concert pianist is entirely different. ;)
I think the main thing is the particular aim of the situation. For example, my daughter has started ballet lessons. Talk about corrections! She's (mostly) fine with them, as she knows they will help her do ballet better and she's there to learn ballet. This was her choice. She knew what she was getting into. Same thing with piano: it's her choice to explore music in this way. If she ever decides she loves it so much she wants to excel at it, then she will seek out those who will be able to refine what she's doing even more.
I do completely agree: There are some things that need to be corrected asap. How she was playing was one of those things. She used to actually have proper curvature, but did not stay long enough at playing piano for it to become a life-long habit, apparently. She was about a week into daily playing this summer when I actually looked at her playing. Eek. My initial dealing of the situation was NOT Montessori-style. When I saw her reaction to my interjection, I realized immediately I had done wrong. I changed how I went about it, stopped correcting her in the middle of what she was doing and instead presented things just before she was about to start or just as she was finishing, etc. Basically, I thought about the best way to meet *her* in all her being. :) What I have done with her may not work with anybody else, I have to admit! lol.
Btw, here's an interesting blog post somewhat on this very topic!