A lot of really good dialogue in this thread, and as I'm reading it I had some thoughts about development.
First an observation. My son is 5 and played t-ball this fall. Not long after Josh Donaldson was on MLB Network talking about his swing, a teammate came to the next game sporting a huge leg kick and drastically different swing. Very obvious that someone watched Donaldson, and went in the backyard with his son to teach his kid to be a big leaguer. But it was entirely too complicated of a swing - way too much motion and coordination for a 5-year old to handle, when they should be taught nothing more than making contact (after the "having fun" requirement is satisfied, but that's a tangent...).
My point is, and I know that a 5 year old and a MLB player are on extreme ends of the spectrum, but what MLB players do shouldn't always be emulated by younger players. What works for a major leaguer doesn't necessarily make it optimal for a college/high school/little leaguer. Different levels of athleticism, development, hand-eye coordination, strength, etc. Cabbagedad has made that point about exit velos of HS vs MLB.
USA Weightlifting has a 3 phase technique development model. Forgive me for not remembering the specifics, but the idea is that early in technique development is when you get your engrained motor patterns that will last forever. Make technical tweaks early on, because it is easier to get out of old habits and train the new pattern. In the later two phases, it takes much more work to correct an incorrect movement. So - as a counter to what I said earlier about not emulating MLB players - do you train younger kids to emulate an MLB swing since it may be difficult to reprogram the movement pattern after years of a slightly different pattern?