The idea is that skill training is really specific, so if you want to get better at a skill you have to practice that skill....exactly. For example say you want to train a throwing movement, maybe throwing a cricket ball. You decide to simulate the movement with a 5lb dumbell. The mechanics of throwing a 5lb dumbell will be very different from those of throwing a ball. When you go back to throwing a ball you are having to learn a new skill.
Doug would say that the rational thing to do is to develop overall strength with weights and then improve your skill at throwing the cricket ball.
This - by the way - is challenging some of the things I've pointed to in the past about the value of "functional" training.
Anyway, I spotted this study that seems to back up this principle - specific resistance training didn't help any more than just skill training.
Effect of specific resistance training on overarm throwing performance.
PURPOSE: The main purpose of this study was to compare the effect of a specific resistance training program (throwing movement with a pulley device) with the effect of regular training (throwing with regular balls) on overarm throwing velocity under various conditions. METHODS: The training forms were matched for total training load, ie, impulse generated on the ball or pulley device. Both training groups (resistance training n = 7 and regular training n = 6) consisted of women team handball players, and trained 3 times per week for 8 weeks, according to an assigned training program alongside their normal handball training. RESULTS: An increase in throwing velocity with normal balls after the training period was observed for both groups (P = .014), as well as throwing with heavier balls and throwing like actions in the pulley device. Although the regular training group seemed to improve more (6.1%) in throwing velocity with normal balls than the resistance training group (1.4%), this difference was not statistically significant. CONCLUSIONS: These findings indicate that resistance training does not surpass standard throwing training in improvement of overarm throwing velocity.