Howard has a saying - I can't remember the exact phrasing, but it's something about having measurable results... maybe Howard can chime in and correct me.
The point is that in order to be efficient at improving something like hitting a softball, you need to start with two very important things. First is a plan on what you actually need to work towards, and second, a tool to measure your progress. I agree with Howard in that Don Slaught, Yeager and Englishbey are teaching quality mechanics. After you study this stuff, you'll see why Epstein is off track. His "fence drill" (yikes!) is a good example of WHAT NOT to do. He means well, but his teachings just really don't make sense for good rotational mechanics.
The RVP "system" is a great tool that, if faithfully used, will dramatically improve your DD's swing mechanics. RVP is not the Holy Grail of hitting. It's simply a TOOL to use as a part of the process. With good knowledge about what good swing mechanics should look like, and a trained eye, it can be a tremendous help. But just because you have the fastest, most powerful car doesn't mean you'll win the race without some knowledge and skill.
P.S. - Siderhawk, that is an awesome post showing the stats. Measurable results. Congrats Howard! WSU and Larabee have taken some lumps, but I think that is one program to watch.