I'm a big fan of Swinging up and hitting the ball in the air but at the Amateur Level there indeed might be a larger Group benefitting from hitting it on the ground vs hitting in the air.
the article linked in the OP article
http://www.hardballtimes.com/g...hitters-best-friend/
states that almost all MLB hitters benefit from hitting it in the air but if you look at hitters with a HR/FB percentage of less than 4% the grounders are actually slightly more productive than FBs and I guess a lot of Amateur hitters hit out less than 4% of their FBs. if you factor in the larger number of Errors at the Amateur Level I can understand why a Coach of a HS hitter that hits only 1 out of 50 flies out of the park wants him to hit it on the ground.
of course the best is still a liner but those slap hitters might be better off aiming for low liners (tendency to miss towards a fly) while the power hitters are better of to aim towards high liners that are bombs when you get slightly under it.
Still I would always teach hitters to drive a ball above the IF but I can understand why some HS coaches teach to rather hit it on the ground.