Ever think that the strike zone should be exactly the same for every hitter. Why should it get bigger or smaller just to compensate for the size or stance of the hitter? Short hitters would have to hit the same pitches that bigger hitters get called strikes. A strike should be a strike, it shouldn't depend on the size and stance of a hitter. It really would change the strike zone by a lot anyway, maybe an inch or two. Might make it a bit easier for an umpire to be consistent. Shouldn't a strike be a strike, just like shooting the ball in the basket? Why should a strike to one hitter be a ball to the next hitter?
I think the strike zone rule is applicable to most, if not all, levels of play. If you try to go to a fixed zone at earlier ages, you run into problems with kids having wider ranging heights (I'm assuming each "level" is going to have to determine is own top/bottom dimensions). I think one problem very young kids have is having to go after the outside ball ("better protect the plate...") and invariably hitting that weak grounder to 2nd base when the ball goes off the last 2 inches of the bat - the really small guys should get a skinnier plate. As kids get a little better, the aggressive ones end up leaning over the plate and the dad invariably screams bloody murder when the pitcher delivers one on the inside of the plate - forcing the batter to hit the dirt (that almost hit him.....you're missing a nice game") I can only assume the umps at early ages are not real keen calling the outside strike when the batter's bat might not even reach out that far in the first place. I am not an ump and not real sure how most of them do their job, but I'd almost want a zone top to bottom that I could envision with a particular batter (the batter gives you a reference point - such reference point gets skewed once they get into a crouch). If I had to imagine a fixed zone and had tall guys and short guys interchangeably, not sure I'd call the zone as well as currently defined.