Well, I tried to get the thread back on the OP topic... if you can't beat 'em, join 'em 
Coach, you know I'm usually with you but disagree on this one. IMO, there is a difference between teaching a different swing and a different swing plane. A different swing plane can be a somewhat minor adjustment suggested to the right type of hitter with the right profile to benefit from a higher launch angle swing. I am more in the camp of others who say it is more detrimental for most HS hitters, for many reasons. This has been discussed at great lengths in other threads. Most HS hitters will benefit from a swing on plane due to both timing and strength concerns as well as a host of secondary reasons including typical defensive capabilities and field conditions.
Sultan, I'm sure I am missing something, but I don't understand your statement ...
... "hitters can't use 30 deg angle with their bat or they would hardly ever hit a 6-10 deg incoming pitch. So hitters must use 10-20 deg swing, which by definition undercuts the ball (giving it the extra 10 deg of launch angle), which therefore creates backspin."
If the incoming pitch is at a 6-10 degree angle and the swing is 10-20 degrees, by definition, wouldn't that result more often in topspin than backspin? I realize that where on the ball contact occurs is also a factor in this equation. Just trying to understand the math of your statement.
Also, you said..
"30 deg (BALL) launch angle takes it out of the park, plus exit speed."
Most HS hitters don't have nearly the strength/bat speed to generate a HR with a 30 degree launch angle. It becomes a routine fly ball out... one of the easiest ways for a defense to get an out. As the hitter progresses in strength, timing, pitch recognition, etc., it may make sense to take the plane more upward beyond pitch plane, depending on the hitter, again, IMO.