Coach Labeots, I use the fence drill as well and so, guess I don't know much. Any breakdown drill is of extreme benefit especially when you are identifying weaknesses in a players swing. bbscout is right in that they need live arm as well. However, the live arm should be to reinforce the correction of flaws and not just swinging a bat. In other words, practicing with a purpose. We have a kid now with a long swing. we are doing all kinds of things to help him. I like the sawed off wooden bat (actually a broken wood bat with the end sawed off) and one hand drills to also teach better hand action for long swingers. I also like the towel drill where the towel is rolled up and put under the front armpit. The towel is released on follow through. It will tell you a lot about the swing. THen again, all of this might be useless info since I have proven I don't know much.
"There comes a time when you have to stop dreaming of the man you want to be and start being the man you have become." Bruce Springsteen